Rodolfo Amedeo Lanciani.

The American journal of science and arts online

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K there was a sufficient carbonic oxyd flame to render the
escaping gases glowing it is evident they would not issue from
the converter as dark smoke, but as incandescent vapor having
its characteristic spectrum. The lack of sufficient name may,
therefore, account for the disappearance of the manganese spec-
trum. The Bessemer flame presents other problems, and opens
an intensely interesting field for scientific investigation ; and by
the use of more delicate instruments than have yet been em-
ployed for this purpose, discoveries may be maae which will
throw new light upon the subject of spectrum analysis.



Art. XXIX. — On a simple method of measuring Electrical Con-
ductivitf'es by m^eans of two equal and opposed magneto^lectric
currents or waves ; by Alfred M. Mayer, PLD.

[Read before the Troy meetiug of the American Association for the Advancement

of Science.]

1, General description of the Method.

A MAGNET is firmly supported in a horizontal position with a
portion of its length projecting beyond a fixed stop (see fig. 2) ;
over this free end of the magnet, and resting against the stop,
are placed two similar flat spirals, formed of the same quality
of copper wire, and having the turns of one spiral in a direction
the reverse of those of the other. The spirals are clamped
together and their four terminal wires are carried vertically
downward into four separate cavities containing mercury ; these
mercury -cups are so connected with a reflecting-galvanometer



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808 A, M, Mayer on ineasuriag Elscirical Condijuttlvitifc.

that, when the spirals are together slid off the magnet, the two
equal electric currents, thus generated, simultaneously tend to
traverse the galvanometer in opposite directions, and therefore
its needle remains stationary. If we now introduce into the
circuit of one of the spirals a resistance equal to that introduced
into the circuit of the other, the needle will still remain at rest
when the spirals are slij)ped off the magnet ; but, if the resist-
ance placea in one circuit is greater or less than that placed in
the other, there will be a deflection of the galvanometer needles
when the spirals are removed. Thus, by introducing wires of
different metals into the circuits we can readily determine their
relative conductivities, by making them of such length that
their resistances are equal ; which condition is attaint when,
on sliding off the spirals, the needle remains absolutely at rest.
If, in the latter case, the wires have equal diameters tnen their
conductivities are directly and their resistances are inversely as
their lengtha

A modification of the above method is discussed in the con-
clusion of this paper ; in which the magnet is replaced by the
terrestrial magnetic force and the spirals and the wires by two
similar coils, from two to three feet in diameter, formed of the
two wires whose conductivities are to be compared. These
coils contain equal lengths of the same sized wires and the same
number of turns ; the direction of the turns being opposed in
the two coils. The coils having been bound together are placed
in a plane at right angles to the line of "the dip," and the four
terminal wires are so connected with the reflecting-galvanometer
that the two induced currents tend to traverse it in opposite
directions. The coils are now quickly rotated through 180**,
around an axis at right angles to the line of the dip, and if the
wires present equal resistances the needle remains at rest ; if it
is denected, the direction and the amount of the deflection
shows which coil has the lesser resistance and affords a means
of estimating the same.

After this general description of the method I will present,
in order, a description of the apparatus used, and of the actions
which take place in it; the degree of precision of the method ;
examples of the determinations of electrical-conductivities, and
experiments on the modification of the method.

2. Description of the Apparatus.

The magnet was formed of a combination of three steel bars,
separated from each other by slips of wood -2 in. thick. The
middle bar was 10*4 in. long and its ends projected '25 in. be-
yond the two side magnets. Each bar was •27 in. thick and "9
m. wide. About three months before this investigation was
undertaken they ha^ been magnetized to saturation by the



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A. AL Mayer on measuring Electrical Conductivities, 809

following process. The axis of a helix, 8*7 ins. long and con-
taining 668 feet of y, in. copper wire, was placed in the line of
" the dip " and a current so sent through it from ten Bunsen
cells that its N. pole was toward the earth. The separate bars
were then drawn through the helix until they ceased to acquire
an increase of magnetism. This method gives a uniform and
powerful magnetization, and probably may be improved by
causing the bars to vibrate as they pass through the helix ;
which can be accomplished by means of a tuning fork ftir-
nished with a long brass stem. After the magnets were com-
bined, as described above, a weight of 1'5 lb. was sustained at
the end of the middle bar.

The magnet was supported in an E. and W. line, 16 '5 in.
above the surfece of the mercury in the cavities of the wooden
" connecting-block" placed below it ; and 2*5 in. of its S. end
projected beyond the wooden clamp which held it

The Spirals were formed of ^V i^ch " double-covered " Lake
Superior wire. Each spiral contained ITeOO in. of wire coiled
in 20 turns, and the terminals were 15*5 in. lonff, thus making
207*06 in. of wire in each spiraL The greatest diameter of the
spirals was 3*9 in. and each had a central opening of 1*7 in.
Their thickness after they were covered with paraffined paper
and varnished was "06 in. The covering of the two terminal
wires of each spiral was saturated with melted paraffin ; they
were then firmly tied together with silken cord to about 4 in.
of their ends where they separated and formed forked termina-
tions.

The spirals were formed in this manner. An iron plate A,
which screws on to the mandrel of a lathe, has cemented on to
its face a disc of hard wood 6, 1*7
in. in diameter and '1 in. thick.
From the center of the plate A
projects a screw e whicn enters
the wooden disc B at e\ When
the plate B is screwed " home "
the disc h fits into the cavity b'
and the plates A and B are sepa-
rated to a distance a little greater
than the diameter of the covered
wire, while the disc b forms a cylinder between them on which
to wrap the spiraL

The end of the wire to be coiled is passed through a hole d
in the plate B, which is then screwed home on to A. The lathe is
then turned so that the wire is coiled over the center disc from
^ A to 6. After the space between the disc is filled with coils,
the free end of the wire is secured and the plate B unscrewed,
while the wire slides through d and the coil is not unwrap-



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810 A. M. Mayer on measuruig Electruxd Oonductivitka.

ped; which would have taken place if it had been coiled
in the direction from 6 to A. The spiral is now saturated with
very fluid paraffin and has cemented on to it, with a hot chisel,
a paper disc previously saturated with paraffin. The spiral is
now removed, the covered side placed against the disc and its
other sur&ce treated in the same manner. The spiral is then
taken off the chuck, and on holding it up to the light the
copper wire is distinctly seen through the translucent covering
of the wire and the paraffined paper cover of the spiral The in-
sulation thus obtained is very perfect and the coils are firmly
cemented together. The termmals are now led radially from
the spirals, and are tightly bound together as described above
To still further strengthen the spirals, both they and their ter-
minals are covered with a firm laver of shellac varnish.

I have thus minutely described the process of making these
spirals for they are of inestimable value in many electrical
researches ; having been used in my recent investigations in
electro-magnetism, and will be again used and referred to in a
subsequent communication.

The galvanometer I specially constructed for this research,
but experience has shown that a coil of shorter and thicker
wire, (say, j\ in. wire in 6 turns) offering less resistance, would
have Deen better than the one employed. The wire of the

f galvanometer was -^^ in. Uiick, and was wrapped around the
ower needle in two layers of 22 turns each ; the opening of
the coil being '15 in. in widtL The needles are 1*66 in. long
and '08 in. diam. The upper needle is -O in. above the lower
with a thick copper plate intervening ; it was not much affected
bv the current m the coil, and was under the influence of a fee-
ble magnet, 18 in. long and 18 in. diam,, which was placed with
its similar poles over the upper needle and 8*2 in. above it
Under these conditions the simple oscillations of the system
were exactly 4^ per minute, ana by lowering or raising the
magnet I could render it more or less astatic The needles were
hung by a frame of fine copper wire to a plane mirror 1*2 in.
square, formed of thin glass silvered by Poucault's prooesB, and
the whole was suspended by a few fibers of unc^un ailk. The
instrument was enclosed in a cover, the front of which was
made of a carefully selected piece of plate glass.

The S. end of the magnet used in inducing the electric
currents in the spirals was 14 in. to the left of the magnetic
meridian line drawn through the point of suspension of the
galvanometer needles and 6 ft. 11 in. distant firom the sama
In this position the magnet caused a deflection of 52' "6 in ihe
needles of the galvanometer and they were brought back into
the meridian by means of the damping-magnet
The deflections of the needles were read off by the beautiful



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A. M, Mayer on measuring Electrical Oonditctivities. 811



method invented by the illustrious Gauss, using a telescope and
scale placed as described below.

The telescope was of '7 in. aperture and 12 in. focus ; just
under its object glass was placed, at right angles to its axis,
a rod of wo6d 1 in. square and 1 meter long, covered with
drawing paper. This rod was divided off into centimeters by
lines 1"*^ thick ; thus, the division lines were ^V of the distance
between two similar .sides of the centimeter divisions. A thick
spider thread was selected which just covered a division line,
and therefore was also, apparently^, 1°*™ thick. By this simple
device, — using one and the same side of the spider line as point
of reference, — ^we can accurately estimate deflections of the
needles corresponding to j\ of a division of the scala

The scale was 2*286 meters distant irom the center of the mir-
ror, and therefore a motion of 1 division of the scale over the
spider thread corresponded to an angular deflection of 7' 80'^
and as we have seen that j\ of a division can be accurately
read, it follows that we can determine a deflection of 46''. In
this paper I will give the deflections in
divisions of the scale, which can be con-
verted into minutes of arc by multiply-
ing them by 7*6.

Connecting-block is the name I give to
the block of wood, placed under the pro-
jecting end of the magnet ; it has four
cavities containing mercury, by means
of which we make the various electrical
connections required in the experiments.
Fig. 2 gives a view of this block and
shows tne manner of making the con-
nections when the object is the measure-
ment of relative electrical resistances.
Four holes, 1 in. in diam. and 1 in. deep,
separated by walls '1 in. thick are bored
out of a block of wood, and then coated
with thick shellac varnish. A, A' are
the terminal wjres of one spiral, B, B'
those of the other. The wires to be
compared are at E and F. If E repre-
sent the standard wire of a fixed length,
then the wire F has to allow of its length
being altered so that its resistance may
be made equal to that of E. This is ar-
ranged by sliding one end of this wire
through a heavy copper clamp (not shown in the fig.) which is
fixed in the mercuiy-cup B, while the other end, previously
well amalgamated, dips into cup A'.




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312 A. M. Mayr on measuring JEleciriccd Conductivities.

The two spirals I are firmly clamped together, so that when
slid quickly off the magnet S, they both have the same direc-
tion of motion. By referring to fiff. 2, the directions in which.
flow the currents, thus produce^ can be readily followei
Takinff spiral A A', the current flows down the terminal A4-
tiirougn the wire E into B', thence by the wire D through the
galvanometer and back by the wire C to A'—, the other 1^ of
the spiral, thus completing the circuit In the case of the spi-
ral B B', the current flows down the terminal B+ through tlie
wire F, thence by the wire C through the galvanometer; re-
turning by D to Dy thus forming the circuit

It is evident that if the spirals by themselves generate equal
currents, and the resistances £ and F are equal, no deflection of
the needles will ensue, for equal and contrary currents will tend
simultaneously to traverse the galvanometer.

The manner in which contacts are made in these experiments
is of great importance. The double silk covering of the wires
is unwrapped to -2 in. from their ends ; the unwrapped silk is
then firmly wound over the end of the silk covering and satu-
rated with thick shellac varnish. The uncovered end of the
wire is now scraped, rubbed with nitrate of mercury, and well
amalgamated, up to the silk covering. The iron wires were
amalgamated by dipping their uncovered ends into sodium-
amalgam. Thus, even if the end of the wire should dip deep-
er than -2 in- into the mercury, the point of contact will yet re-
main at that distance from the end, as the shellac prevents con-
tact above the amalgamated portion of the wire. The termi-
nals of the spirals and of the galvanometer coil were formed in
the same manner. One end of the wire whose resistance was
to be compared to the standard copper wire, was uncovered and
well cleaned for some portion of its length, so that it could be
drawn through the heavy copper clamp ui^til its length equal-
led in resistance the standard wire. The wires were then re-
moved and their lengths accurately measured.

3. Investigation into the actions which take place in the apparatus.

In the general introductory description given of the method, I
have, for simplicity of illustration, assumed that when the two
spirals, — similar as to form, length of wire and resistance, — ^are
slid off the magnet, no current would be sent through the gal-
vanometer. But this cannot be, for the hinder spiral is furmer
on the magnet than the other by 06 in. and therefore cuts more
"lines of magnetic force," and also, the two spirals traverse
simultaneously portions of the field differing in magnetic in-
tensity.

The following experiments will exhibit the above action. I
will call the back and fix>nt spirals respectively A and B.



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A. M. Mayer on measuring Electrical Conductivities. 318

They were reversed on each other, clamped together and had
their terminals dipping in A and A' so that the two induced
currents would tend to traverse the galvanometer coil in oppo-
site directions.

The mean of three experiments shows that when the spirals
are slid off there is a deflection of 248 div. in favor of spiral A.
But, on making spiral B the back spiral, the needle moved 8*9
div. in its fevor ; thus showing that spiral B offers less resistance
than A, though both lengths were contiguous pieces taken from
the same sample of wire. On again placing A against the stop,
1 found that a resistance of 2*9 in. of y', in. wire, when attached
to one leg of this spiral, reduced its action to ecmal the forward
spiral and the needle remained absolutely unaffected when the
spirals were quickly removed from the magnet

The following experiments show the effects of separating the
spirala The balanced spirals remaining, in other respects, as
in the last experiment, I separated them 05 in. by intervening
card-board ; the needle was now deflected 105 diV. in favor of
the back spiral, and on increasing the separation to 125 in, the
action of tne back spiral equalled 8*6 div. of the scala

I have said above that the two opposed currents tend to
traverse the galvanometer coil, because theoretic considerations
induce me to hold the opinion that two currents cannot simul-
taneously traverse a wire in opposite directions, and that onlj
the excess of the intensity of one current over the other is
really propagated through the wire.

The next point to be considered is the mutual inductive ac-
tion of the spirals. The directions of the turns in the spirals
being opposed, and as the current in each, on sliding them off
the magnet, rises rapidly to a maximum intensity and as
(juickly comes to 0, it follows that they must exert a mutual
inductive action. But although the current in one spiral during
the rise to its maximum causes an induced current in the other
spiral in the same direction as that induced in it by the magnet
yet, as the current decreases as quickly to after it has reached
its maximum, it follows that a current in the opposite direction
to that induced by the magnet in the other spiral will now
quickly follow it, and as these currents, -f , and — are equal,
tnere will be no increased outside effect arising from their inter-
action ; and many experiments showed that whether a copper
disc was placed between the spirals or an equally thick disc of
paper, the action at the galvanometer was the same.

The following experiments on this subject appear to confirm

the above view. Tne two spirals were placed on the magnet^

but only the front one was connected with the galvanometer,

while the terminals of the back spiral were separated so that no

Am. Joub. 8ci — Second Sbribs, Vol. L, Na ISQT— Nov., 1870.

90



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814 A. AC Mayer on measvring Mectrioai (hnditctivities.

current went through it when the spirals were together slid off
the magnet The action of one spiral alone was sufficient to
deflect the galvanometer needle about 60^. This deflection was
reduced to 1** 18' by placing in its circuit a helix of 785 ft of
no. 18 copper wire ; the mean of six experiments (the range of
which was only jV div.) giving 104 div. The ends of the
back spiral were now so connected that an equal current flowed
througn it in a direction the reverse of the other. The mean
of six deflections, produced by sliding together the spirals off
the magnet, equalled 104 div., the same as in the previous ex-
periment ; thus showing that the mutual inductive action of th^
spirals had no effect on the intensity of the induced magneto-
electric currents.

It was also found that on passing the induced current from a
spiral through another spiral on which rested a third spiral
whose ends were connected with the galvanometer, that no de-
flection ensued when the magneto-electric current was passed
through the inducing spiral

However, the magneto-electric currents were of such low in-
tensity that probably thejr were not able to produce an induced
current in tne second spiral capable of d^ectin^ the needle,
and that therefore the experiments here narrated are of little
value ; nevertheless, I think the reasoning given above will be
supported by experiments made with more powerful magnets
and with laiger spirals.

4. 2%€ degree of PrecUum of the method.

The d^ree of precision of this special apparatus was deter-
mined in the following manner. A copper wire 123 ins. long
had opposed to it a resistance which was about eaual to 120
in& of its length and the mean deflection of the galvanometer-
needles was carefully determined The copper wire was now
shortened 1 in. and the deflection again determined ; this was
repeated, — determining the amount of deflection produced after
each shortening of 1 in., — ^until 6 in. had been cut off. These
experiments showed that a diminution or increase of resistance
of T^T P8J^ ^ ^^^ of the wires caused a deflection of 4 div. of
the scale, or of 8' of arc, in the galvanometer-needles. But we
have seen that 1 div. can be read on the scale, therefore, we
can, with this special apparatus, detect and measure an increased
or diminished resistance of 7 jy part But as the galvanometer
can be removed to even twice the distance at which we read its
deflections, I think I am safe in saying that with this method,
as applied with the above apparatus, we can measure a dif-
ference of resistance in two conductors of y Jy part ; which is far
within the variations observed in different samples of wires of
the same lengths and diameters.



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A. M. Mayer on measuring Ekctrical Qmductivities. 816

If a galvanometer formed of 6 or 8 turns of *1 in. wire were
used in connection with a powerful magnetic battery and larger
spirals of thicker wire, while the galvanometer is placed at a
greater distance, I have no doubt tnat a variation of ttV? P^^
can thus be detected and measured.

5. McampUa of the determinaUone of electrical conducHvities hy

this method.

The object of these determinations was not to furnish science
with new and accurate data, — ^for that would have required a
careful personal supervision of the operations of preparing chem-
ically pure metals, — ^but it was to give examples setting forth
the practice of the method.

I had prepared " hard-drawn " wires, of No. 18 B. W. G.
(=•049 in. diam.), of copper, silver, iron, and German silver.
These wires were found to have the same diameter. They were
all covered with a double wrapping of silk.

Silver. — ^The spirals were balanced, by the introduction of an
increased resistance in the back-spiral, so that no deflection took
place on sliding off the spirala A length of 120 in. of the sil-
ver wire having been placed in the circuit of one spiral, it was
found that 127 in. of copper wire were required in the other
circuit, in order to equal it in resistance. Taking the copper
wire as the standard of comparison, at 100, we have

127:120::100::94-4a

Matthidssen (PhiL Trana, 1858, 1862) makes the ratio of the
conductivity of silver to copper, both hard-drawn, as 100: 99*95
or about equality ; but in my determination the silver is 5*5 per
cent below the copper. I therefore suspected impurities in the
silver, and an examination of the wire kindly made by my col-
league, Dr. Wetherill, showed that it contained about 01 per
cent of gold and a trace of iron. This accounts for the low
number found, and affords a good illustration of Pouillet's re-
mark, that the purity of a metal is most readily determined by
a measure of its electrical conductivity. The electrical test of
purity, however, exceeds in delicacy the ch^nical examination ;
for a very minute percentage of allov causes a great increase of
resistance, and if we could be sure that the wire«i we compared
were in the same physical condition as to annealing or nsuxi-
ness, we could probably use this method as a means of deter-
mining the percentage of a hnovm metal which formed the alloy.
Pouillet shows Cmit^ de Physiaue, 1866, yoL i, p 606J that
silver whose conductivity is 100 wnen pure, is only 51 when it
contains 087 of alloy, and is 47, 42 and 89 when it contains
respectively lOO, 148, and -258 of alloy. Pure gold gave 89,
but 049 of alloy reduced its conductivity to 18 ; and Jenkin



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816 A M, Mayer on measuring Electrical ConducUvities.

has found that an alloy of 1 part of silver and 2 of gold pre-
sents almost as much resistance as German silva*.

Iron, — ^The three following determinations were made of the
conductivity of the best quality of iron wire relatively to the
standard copper wire.

1) The resistance of 240 ixL of copper wire = 36*7 in. of iron wire.

2) *• " " 111*6 " •* " *• ^ 16*16 •* ** " "

;\ u (t it 00 ti 11 a u 22 g.gi^ u u (I a

GKving for the relative conductivity of iron,

1) 240 : 36*7 = 100 : 16*29

2) 111*6 : 16*16 = 100 : 14*48
;3) 60 : 8*67 = 100 : 14*46

14*74 s Mean.

E. Becquerel (Ann. de CL et Phys., Ill, xvii, 266) gives 13*6
for the conductivity of iron, copper being 100 ; and both wir^
hard-drawn ; while Matthiessen ^PhiL Trana 1858, 1862) deter-
mines 16*81 as the conductivity of iron, copper being 100, and
both hard-drawn.

The mean oi Becquerel and Matthiessen = 15*20
My determination = 14*74

Difference = '46



Online LibraryRodolfo Amedeo LancianiThe American journal of science and arts → online text (page 89 of 109)