resistance thermometers and lamp filaments, may have very small
resistance spools, while bridges for general work and where alloys
326 MEASURING ELECTRICAL RESISTANCE [ART. 1403
are to be measured should have larger spools. In this latter case
a pointer galvanometer of 0.5 megohm sensibility will serve for
most requirements.
The absolute precision of a resistance measurement must pri-
marily depend upon the accuracy of the standards. The spools in
a Wheatstone bridge become for the time being the standards
employed. It is always possible, if one has one standard resist-
ance, the precision of which is known, to check up or calibrate
a Wheatstone bridge so that the true resistance of all its coils
becomes known. The systematic procedure for doing this will
generally be given to customers by the makers of the bridges
or for a relatively small fee a bridge will be tested and certi-
fied to by the National Bureau of Standards.
No resistance measurement, however, can be considered as very
precise unless careful attention is paid to the magnitude of the
measuring current and the temperature of the surroundings in
which the sample is placed at the time it is measured. For much
work the watt capacity of the bridge spools will exceed that of
the samples and the magnitude of the current must be adjusted
to the latter.
In general, it is considered very good work if a Wheatstone bridge
will measure resistances from 10 to 10,000 ohms with a precision
of 0.02 of 1 per cent. Most Wheatstone-bridge work will range
about 0.05 of 1 per cent.
1403. Resistance Standards. In considering resistance units
attention should be drawn to the distinction between resist-
ance units used for standards of resistance and resistance units
which are intended to carry considerable current when used with
potentiometers and like instruments. These latter should be
spoken of as current-resistance standards. Resistance standards
proper do not need to have much watt-dissipating capacity.
Their requirements are unchangeableness with time, low tem-
perature coefficient, and small thermo-electromotive force against
copper. They should be susceptible also of immersion in kerosene
oil, so their temperature may be accurately ascertained. The
development of resistance standards has passed thru quite an
evolution, and the latest type as designed by Dr. E. B. Rosa and
endorsed by the National Bureau of Standards, will now be briefly
described.
A paper entitled "The Variation of Resistance with Atmospheric
ART. 1403] SOME GENERAL PRINCIPLES CONSIDERED 327
Humidity" was read before the American Physical Society at
Washington, April 21, 1907, and later published in the Bulletin of
the Bureau of Standards, Vol. 4, 1907-8, page 121. In this
paper by E. B. Rosa and H. D. Babcock, it was demonstrated
" that the shellac " (covering the wire of wire-wound resistance
spools) " absorbs moisture from the surrounding atmosphere and
expands, stretching the manganin wire and thereby increasing
the resistance." This variation in resistance under circumstances
exceptionally favorable for stability may amount, in different
seasons of the year, to as much as 2 parts in 10,000, and under less
favorable circumstances to 7 or more parts in 10,000.
The discovery of this property of shellac of swelling with mois-
ture and straining the wire has led to the present type of con-
struction of resistance standards adopted by the Bureau and
executed by prominent instrument makers. The construction
referred to is fully described by Dr. E. B. Rosa in a paper entitled
" A New Form of Standard Re-
sistance," and published in the
Bureau of Standards' Bulletin,
Vol. 5, 1908-9, page 413. The
essential feature of the construc-
tion consists in hermetically sealing
the brass cylinder, upon which the
shellacked resistance wire is wound,
in a metal cylinder filled with kero-
sene oil. In this manner all access
of moisture to the resistance wind-
ing is effectually prevented. This
construction so improves the con-
stancy of the resistance that of 28
coils kept under close observation
there was " only one coil of the
above twenty-eight that has changed
as much as two parts in 100,000
during the past twelve months" *
This type of standard, for re-
sistance units of 10 ohms or more
without potential terminals, is shown in section in Fig. 1403a.
" The new form of resistance standard is much smaller than the
* Bulletin, Vol. 5, page 427.
FIQ
328
MEASURING ELECTRICAL RESISTANCE [ART. 1403
Reichsanstalt type, so long and so favorably known throughout the
world as a standard of resistance. It weighs only about 400 g
filled and measures only 7.5 cm across the terminals instead of
16 cm. For measurements up to an accuracy of .001 per cent
it is measured as it stands, its current capacity being ample when
using reasonably sensitive galvanometers, and the small ther-
mometer in the central tube giving its temperature with all needed
accuracy. The temperature coefficient is generally not greater
than .002 per cent per degree, so that a quarter of 1 degree un-
certainty in the temperature would cause an error less than that
allowed. *******
FIG. 1403b.
" The apparatus employed in comparing the coils of any denomi-
nation with one another is shown in Figs.' 7 1403b and 1403c. "It
is a Wheatstone bridge with the coils arranged in the circle, having
two extra coils inserted, on which shunts may be applied for
balancing the bridge instead of shunting directly the ratio coils.
Or these two openings may be closed by heavy links and the
shunts applied directly to the ratio coils. The circular frame is
so hinged that it may be opened far enough to admit a larger
coil, as, for example, one of the Reichsanstalt form, which may thus
AET. 1403] SOME GENERAL PRINCIPLES CONSIDERED 329
be directly and conveniently compared with one of the new Bureau
of Standards form, Fig." 1403c. "The apparatus is very conven-
ient, is compact, requires a relatively small oil bath, or may be used
without an oil bath except in comparisons of extreme precision,
.and will accommodate any kind of a resistance standard that is
provided with terminals for dipping into mercury cups."
FIG. 1403c.
When a resistance standard is 1 ohm or less it is necessary to
provide it with potential terminals. This leads to a different
construction than that adopted for the higher denominations.
The resistance material for these low-resistance standards is in the
form of heavy manganin wire or sheet which is too massive to be
strained by the swelling of shellac. In Fig. 1403d is shown a
0.001 -ohm standard resistance of the Reichsanstalt form.
This form of standard is guaranteed by its makers to an accu-
racy of 0.02 of 1 per cent but may be made more accurate.
" When immersed in oil and used as a precision standard of resist-
330
MEASURING ELECTRICAL RESISTANCE [ART. 1403
ance it has a current-carrying capacity of 32 amperes. Used
for measuring current to a lesser degree of accuracy it will carry
100 amperes or more."
FIG. 1403d.
Otto Wolff of Berlin supplies a line of standard resistances
which have the following denominations and current-carrying
capacities when used for precision measurements and as current
resistance-standards .
Resistance,
ohms
Current capacity for
precision measurements,
Current capacity as
current-resistance
amperes
standards, amperes
100,000
0.003
0.01
10,000
0.01
0.03
1,000
0.03
0.1
100
0.1
0.3
10
0.3
1
1
1
3
0.1
3
10
01
10
100
001
30
300
0.0001
100
1000
ART. 1404] SOME GENERAL PRINCIPLES CONSIDERED 331
These standards are called " Small Pattern " standards. They
are all arranged to be dipped into mercury cups by means of copper
terminals which in all types are separated the standard distance
of 16 cms from center to center. The resistance proper is mounted
inside a brass case with a hard rubber top which carries the ter-
minals. The case is perforated to permit free circulation of oil
when the standard is immersed in a bath of petroleum. The
general rule may be applied that, when used in oil for high pre-
cision work, a load of 1 watt is allowable, while for less accurate
work a load of 10 watts may be applied.
1404. Resistance Boxes and Wheatstone Bridges; General
Remarks. In determining the value of one resistance in terms
of another it is very desirable to have a series of known values of
resistances which can be varied thru a wide range by known
amounts. The fundamental purpose of all resistance boxes,
whether employing plug contacts or dials, is to provide means for
obtaining the largest possible number of values from the fewest
possible number of known resistance units. The very varied
forms of construction that instrument makers have given to re-
sistance boxes, have had, more or less, the above object in view.
Where the number of coils or units is made greater than the least
number required for varying resistance thru a given range, it is
done to serve some purpose of convenience of working, or to in-
crease the facility with which .resistance values may be changed
and mentally added up.
The advantages of being able to obtain a specified number of
resistance values with a minimum number of resistance units
pertain both to the user and to the maker of the set. The ad-
vantages to the user are reduced cost, a smaller number of coils to
get out of adjustment and to measure when checking up a set,
economy of space, a smaller number of contacts and, often, in-
creased simplicity in forming combinations of resistance values.
The advantages to the manufacturer are in the nature of re-
duced cost of construction which must, of course, be reflected to
the purchaser.
In short it is because of the gain in the above particulars that
it is desirable to combine a small number of units of resistance to
obtain a great many values. If it were otherwise there would be
no occasion for the construction of resistance boxes and one would
employ a separate coil for every value which he might wish to use.
332 MEASURING ELECTRICAL RESISTANCE [ART. 1405
The various methods that have been devised for combining re-
sistance units have been discussed already and it may only be
remarked here that the so-called decade plan is considered so
very superior to all others that its further adoption is urged.
1405. Watt Capacity of Resistance Units. The current-
carrying capacity of a resistance unit in a resistance set will de-
pend not only upon the ability of the unit to dissipate heat but
also upon the ohmic resistance of the unit. It is therefore meaning-
less to speak, as is often done carelessly, of the current capacity
of coils. The watt-dissipating capacity of a coil or unit is, on the
other hand, a definite matter, and in a well-constructed resistance
set will be the same for each coil in the set. The watts which a
spool will dissipate will be approximately proportional to the
square of the voltage at the terminals of the coil divided by the
resistance of the coil; or it will be approximately proportional to
the square of the current thru the coil multiplied by the resistance
of the coil. No definite statements can be made of the number
of watts which each coil in a set may safely carry. Often a tem-
perature which will not injure the insulation will permanently
impair the precision of the coil. The manganin wire of which
resistance coils are mostly wound is generally permanently
diminished in resistance probably by the release of the molecu-
lar strains in the wire when the temperature is considerably
elevated. To make this effect as small as possible resistance coils
should be artificially " aged " before their final adjustment by
being given a prolonged baking (from 24 to 48 hours) at a tem-
perature of from 130 to 140 C. It is, therefore, unsafe to load a
precision resistance box so that any of its spools attain a temper-
ature above that at which they were aged. A general rough rule
for resistance boxes, the spool windings of which are on brass, is to
limit the watts per spool to 3 watts. Thus one should limit the
v 2
current thru the spools so that is not greater than 3, v being the
volts at the spool terminals and r the resistance in ohms of its
winding. In using a Wheatstone bridge, in which the resistance
of the ratio arms at any moment may be one time 1 ohm and
1000 ohms, and then again 1000 ohms and 1000 ohms, it is well
to keep from 10 to 100 ohms in the battery circuit ali the time.
An E.M.F. may then be employed which, acting thru this resist-
ance, will not overheat the low-resistance coils.
ART. 1406] SOME GENERAL PRINCIPLES CONSIDERED
333
In the best boxes the windings are not only upon brass spools
but these are in direct metallic connection with the brass blocks
upon the top of the box. By this arrangement much of the heat
developed is conducted to the brass blocks and there dissipated.
Some boxes are provided with outside cases of perforated metal
so the entire set may be immersed in kerosene oil. A set used in
this way will have the watt-dissipating capacity of its coils in-
creased several times.
It is rare, however, in using a resistance set for the measurement
of direct-current resistance that one has any occasion to reach,
even approximately, the watt capacity of the set. Plug and dial
rheostat boxes and sometimes the rheostats of Wheatstone-bridge
sets are employed as auxiliary apparatus and are required to
carry as great a load as is allowable. For this reason chiefly it is
desirable, in selecting resistance sets, to require that their heat-
dissipating capacity shall be as large as practicable without undue
increase in the size of the sets.
1406. Construction of Resistance Spools. The type of re-
sistance spool shown in the illustration, Fig. 1406, combines those
features which experience has shown to be desirable. The wind-
ing is upon a brass tube. The wire is
chosen of such a size (whenever this is
possible) that the required resistance
is obtained by winding it in a single
layer which extends the entire length of
the spool. The brass spool is mounted,
with good metallic connections, upon
the brass shaft, the upper end of which
serves as a stud for the brushes of a
dial switch to rest upon. The winding
itself is of manganin wire and is wound
bifilar. This wire is chosen because of
its high specific resistance (about 26
times that of copper), small tempera-
ture coefficient (about 0.00002 per
degree C.) and small thermal E.M.F.
against brass or copper. There are other
materials which would probably meet the above conditions quite
as well but they are of recent development and have not as yet had
the thoro trial and endorsement which has been given to manganin.
Brazed
Copper
Soldered
FIG. 1406.
334 MEASURING ELECTRICAL RESISTANCE [ART. 1407
If the spools are 100 ohms or less in resistance the terminals
of the resistance-wire proper are not soft soldered directly to the
brass. A short length of copper wire is silver soldered to the
manganin and this terminal is in turn soft soldered to the brass.
In making the soft-soldered connection of the copper terminals, a
small variation in the length of the copper terminal affects very
little the total resistance of the spool. Thus, adjusted spools may-
be soldered in place in resistance sets without the necessity of
making a further resistance adjustment after the spool is in place.
The prevailing practice is to cover the outside of the spools with
a thick covering of shellac which is baked hard and is glossy. As
shellac has been shown to absorb moisture from the air and to
strain the wire by swelling, a non-hydroscopic material should
be sought. If the proper material were found the permanent
precision of resistance sets would be considerably improved.
Spools constructed and mounted as above may be made to
dissipate safely between three and four watts each. In boxes of
cheaper grade the manganin is wound upon wooden spools. These
are inferior in heat-dissipating capacity and other respects. When
of the same size as the brass spools their watt-dissipating capacity
is between 0.25 and 0.5 watt or about 0.1 that of a brass spool
mounted as above.
1407. The Precision of Coils in Resistance Sets. The pre-
cision of coils in resistance sets is not usually required to be so
high as that of individual resistance standards. Even if the
adjustments were made originally as high as that of the best
individual standards they would only hold for a particular tem-
perature. Furthermore, as different sizes of wire must be used
in the same resistance-set, it is impracticable to select all the sizes
of exactly the same temperature coefficient; and thus exact tem-
perature corrections for all the coils in a set, for other tempera-
tures than that at which they were adjusted, are hardly possible.
The coils, moreover, not being hermetically sealed in oil will not
maintain the high precision which might be given to them and
which pertains to resistance standards. The usually unknown
resistance of leads, the contact resistances under binding posts,
and the various plug or brush contact resistances which enter in
a resistance set is another reason why the highest possible pre-
cision is not demanded. The best makers guarantee conserva-
tively the precision which pertains to the resistance sets which
ART. 1408] SOME GENERAL PRINCIPLES CONSIDERED 335
they list. A Wheatstone bridge of the highest grade, intended for
precision work, will have the following guarantee: The rheostat
coils will be guaranteed to an accuracy of 0.02 of 1 per cent, with
the exception of the 0.1-ohm coils which will be guaranteed to 0.1 of
1 per cent and the 1-ohm coils to 0.04 of 1 per cent. The coils hi the
ratio arms of such a set will be guaranteed to be adjusted to an
accuracy of 0.02 of 1 per cent and to be like each other to 0.01 of 1
per cent. A Wheatstone bridge of more moderate precision and of
about one- third the cost would be given an accuracy of adjustment
of the coils in the rheostat of 0.05 of 1 per cent and in the ratio
arms of 0.02 of 1 per cent. The very cheapest boxes and Wheat-
stone bridges should not be less accurate than 0.2 of 1 per cent in
their rheostat and 0.1 of 1 per cent in their ratio arms.
1408. Some Features of Outside Construction. The classical
method, for many years the only one employed, of making the
FIG. 1408a.
desired combinations of resistance coils,' is to use brass blocks
mounted on hard rubber plates, which are connected by plugs
inserted between them. In recent years other methods have come
more and more into favor, especially where it is required to obtain
the desired values rapidly. Among such of special value is the
method of dial contacts.
The chief advantage in the use of plug contacts consists in the
great number of combinations which can be effected with com-
paratively few blocks and plugs. The advantage has long been
claimed for plug contacts that this form of contact has the least
resistance. This advantage, however, cannot be allowed for plug
336
MEASURING ELECTRICAL RESISTANCE [ART. 1408
contacts if certain other forms of contacts are correctly designed
and constructed.
In resistance sets of the plug-type it is desirable to use a correct
type of plug. This should have a proper taper to fit the holes
between the blocks. The hard rubber head should be easy to
grasp without hurting the fingers and should not work loose.
The type of plug designed by the author, and shown in Fig. 1408a
meets these conditions and has been much used on high-grade sets.
Brass blocks when used should be of sufficiently heavy con-
struction to permit undercutting so the base of the blocks will be
separated to give sufficient surface insulation over the hard rubber
top of the resistance set. This is important because hard rubber,
which is exposed to light for a long time, deteriorates in its sur-
face insulation.
FIG. 1408b.
In Fig. 1408b is shown a high-grade Wheatstone-resistance set of
the plug decade type which is intended to embody all of the best
and latest features of Wheatstone-bridge construction. This set
is of the four-coils-to-the-decade type as described in par. 503.
An extra coil is added to the decade of lowest denomination. This
extra coil is added to enable the set to be checked up. It is pos-
sible to compare every coil in a set with every other coil in the set,
and to completely intercheck the different decades.
A particularly fine type of a five-dial Wheatstone bridge, con-
structed for the author, embodies the following special features
ART. 1408] SOME GENERAL PRINCIPLES CONSIDERED 337
of construction: The case of the bridge is made of glass which
gives a full view of the coils and inside wiring. The rheostat
consists of five dials, the one of lowest denomination reading in
0.1 of an ohm. The ratio arms can be plugged to give ratios 1 to
10,000 or 10,000 to 1 with many other ratios lying between these
extremes. The four-coils-to-the-decade construction is used in
the rheostat (see 503) and Schone's arrangement is used in
the ratio arms (see 509). The accuracy of adjustment of the
coils in this bridge is better than 0.02 of 1 per cent in the rheostat
and still better in the ratio arms.
FIG. 1408c.
The form of dial construction which perhaps has met with most
favor is the one used by Otto Wolff of Berlin in all his resistance
sets and potentiometers. A top view of an Otto Wolff five-dial
Wheatstone bridge is shown in Fig. 1408c.
Dial switches are only employed for the rheostat, the values in
the ratio arms being obtained with plugs inserted in blocks.
CHAPTER XV.
DEFLECTION INSTRUMENTS AND GALVANOMETERS.
1500. Distinction Between Indicators and Deflection Instru-
ments. We shall discuss these chiefly from the standpoint of
their employment in resistance measurements.
As resistance measurements may be broadly divided into de-
flection methods in which the value sought is determined in
terms of the deflections obtained with a deflection instrument
- and null methods in which a current detector is used
merely to indicate when no current is flowing in its circuit
the instrumental requirements for the two methods are quite
dissimilar.
For deflection methods the instruments used are voltmeters
and ammeters, pointer galvanometers, and mirror galvanometers
with telescope and scale or lamp and scale.
When deflection instruments, voltmeters and pointer galva-
nometers are used for measuring resistance it is required that the
pointer should accurately return to zero after being deflected and
that the scale be as long as practicable. This should be either
carefully calibrated for correctly measuring equal increments of
current, or if the scale divisions are equally spaced the deflections
should increase proportionally with the current. If the above
requirements are met, then the possible precision obtainable will
increase with the number of smallest divisions marked upon the
scale.
The damping of the system of such a deflection instrument
should be such that the deflection will reach quickly its maximum
value without any oscillation. It is much easier to take close
readings when the measuring current is not very steady if
the instruments are aperiodic or critically damped.
It is not generally required that deflection instruments, when
used for measuring resistance, shall have a very high sensibility, ex-
cept when used for insulation testing by direct deflection methods.
A good voltmeter of the Weston type will have about 100 ohms
338
ART. 1500] DEFLECTION INSTRUMENTS 339
to the volt and consequently, for full scale deflection, will take
about TT-7T = 0.01 ampere. A current of this magnitude will
JLOjUUU
cause very little heating in any resistance which is measured
tho even this small current will alter the resistance of a tungsten
lamp-filament which, consequently, cannot be measured accurately
with a voltmeter. One need for sensibility in such an instrument
is to be able to use so small a measuring current that resistances
will not be appreciably heated.
Portability is a feature which is generally sought and is, of
course, obtained with all instruments of the voltmeter or am-
meter type.
For measuring insulation resistance by direct deflection specially
designed and constructed galvanometers which have high sensi-
bility and proportional deflections are required. These high
sensibility insulation-testing galvanometers were formerly of the