Copyright
Wirt Robinson.

The elements of electricity online

. (page 31 of 46)
Online LibraryWirt RobinsonThe elements of electricity → online text (page 31 of 46)
Font size
QR-code for this ebook


cent. This may be reduced by connecting the voltmeter, as shown
by the dotted line D, in shunt with both the lamp and the am-
meter, or by reading the ammeter before the voltmeter circuit is
closed. In a similar manner it may be shown that if the current
be large and the difference of potential between A and B be small,
the connection as shown in the figure is the best.

The determination of power from the reading of two separate
instruments does not give correct results when applied to alternat-
ing current circuits. This fact cannot be explained until the
subject of alternating currents is reached.



ELECTRO-MAGNETICS.



391



498. Measurement of Power by Electro-Dynamometer. By

making a slight change in the connections of an electro-dynamom-
eter, it is possible to use that instrument to measure electric
power. For example, suppose we wish to measure the power ex-
pended in an incandescent lamp. Connections are made as shown




Fig. 240.



diagrammatically in Fig. 240. F represents the heavy wire fixed-
coil of a Siemen's electro-dynamometer (see Fig. 171). The end
of this coil connected to the terminal A is left undisturbed. The
other end, which was fastened to the metal bracket at D, is discon-
nected and attached to G, one terminal of the lamp. The main
current now enters at H, passes through the lamp and the coil F
and out by A. A current is shunted off at H, passes through a
resistance R of several thousand ohms, thence to the terminal (7,
thence around the movable coil M to the bracket D and finally
reunites with the main current at G.

In Par. 383 it was shown that the force exerted between the two
coils carrying currents is f=KII f

K being a constant and /

and I' being the currents in the respective coils. If E be the dif-
ference of potential between H and G, and if R be the resistance
of the shunt circuit HRCMDG, then the current through the
movable coil is I' = E/R. Substituting this for I' in the expres-
sion above, we have -^



whence, since K and R are



392



ELEMENTS OF ELECTRICITY.



constants, we see that the force exerted between the two coils of
the instrument is proportional to IE, the watts consumed between
H and G. In the same paragraph it is shown that this force is
proportional to the angle of torsion, that is, to the angle through
which the milled head of the dynamometer is turned in order to
bring the pointer of the movable coil back to the zero, whence the
wattage between the points H and G is also proportional to this
angle.

As pointed out in Par. 497, an error is committed in this meas-
urement unless the shunt current (the voltage current) be so small
as to be negligible. On this account, the resistance R is made very
large.

499. Indicating Wattmeter. Instruments from which may be
read direct the power developed between two points in a circuit




Fig. 241.

are called wattmeters. They are of two general classes, the first
giving the value of the power at any instant and called indicating
wattmeters; the second summing up or integrating these instan-
taneous values and called integrating wattmeters.

Indicating wattmeters operate on the principle of the electro-
dynamometer described in the preceding paragraph, but are usual-
ly so arranged as to avoid the errors pointed out in Par. 497. Fig.
241 represents the external appearance of the Weston wattmeter.
Across the top there are three terminals, the outer ones being used



ELECTRO-MAGNETICS.



393



for the voltage current and one of them being marked -f . The
central terminal is used in a certain process of calibration, not
necessary to describe here. On the left side are the two terminals
for the main current, one of these also being marked +. At the
bottom is a button switch which closes the voltage circuit. This
instrument may be used with either direct or alternating currents.
When in use, if the main current be brought in at the plus terminal,
the voltage current must enter by the plus terminal of the top
row; if the main current be brought in at the negative terminal
the voltage current must enter by the negative terminal of the
top row.

The internal arrangement of the instrument is quite similar to
that of the D. C. A. C. voltmeter as described and figured in Par.
470. Fig. 242 represents diagrammatically the connections made




Fig. 242.

to read the power consumed in two incandescent lamps in series.
The main current enters at A, passes around the two fixed coils
in the direction shown by the heavy arrows, and leaves by B. It
does not pass through the movable coil. The voltage current is
shunted off at C, enters at D, passes through the large resistance
R, which is wrapped so as to be free from inductance, thence to
the movable coil around which it flows as shown by the broken
arrow, then around the fixed coils but opposite in direction to the
main current, thence out by E, and rejoins the main current at F.



394 ELEMENTS OF ELECTRICITY.

The current through the fixed coils, as has already been pointed
out (Par. 497), is greater than the current through the lamps
since it consists of that current plus the shunt current. To
correct for this, the shunt current is carried around the fixed coils
in opposite direction to and making as many turns as the main
current.

500. Integrating Wattmeter. A consumer of electrical power is
charged on an equitable basis when he pays in proportion to the
work performed for him by the current. He must therefore pay,
not for the power alone, but for the product of the power and the
time during which it has been supplied, for since power is the
rate of doing work = w/t (Par. 492), work is equal to power X time.
Electrical power is therefore sold not by the watt, but by the watt-
hour, or more usually by the kilowatt-hour (Par. 496). The
wattmeter described in the preceding paragraph indicates the
instantaneous values of the power but takes no account of the
time element. Instruments which sum up the successive amounts
of work performed by the current are called integrating wattmeters.
Their principle is simple but can not be fully explained at this
point. One form consists of a coil which revolves continuously
as long as the current flows through it, the rate of revolution
at any instant varying directly with the power, and therefore
the total number of revolutions varying with the total amount of
work performed. These revolutions are recorded by an arrange-
ment like that used in cyclometers but the dials are graduated to
read kilowatt-hours direct. The instrument is therefore anal-
ogous to a gas-meter which indicates at any instant the total
amount of gas which has flowed through it up to that time but
does not indicate the amount actually flowing through.

501. Electrical Transmission of Power. The two prime sources
of power utilized at present are water and steam. Of these, water
power is much the cheaper.

The difference in level, upon which water power largely depends,
may be natural, as in the case of falls, or may be artificially
produced by the erection of dams. In either case, unoccupied
localities suitable for the development of such power are rapidly
becoming scarce. In the immediate vicinity of these falls and
dams, the available sites for power plants are usually restricted.
By means of shafting, belting, cables, etc., the power developed



ELECTRO-MAGNETICS. 395

by these plants may be transmitted a few hundred feet. Beyond
this limited zone, recourse must be had to steam power.

In the majority of steam plants, coal is the fuel used and this
has to be transported from the mines to the plants. On the aver-
age, the cost of transportation is greater than the cost of the coal
itself, therefore, steam plants located near the mouth of a coal
mine have a great advantage over those at a distance.

From the foregoing, the need of a method of cheap transmission
of power to a distance is evident. This problem is solved by elec-
tricity, the mechanical power developed by the plant being trans-
formed into electrical power, sent out over the line to the desired
spot and there transformed back into mechanical power.

502. Considerations Affecting Electrical Transmission of
Power. It was shown above (Par. 494) that the electrical power
between two points in a circuit is measured by IE, the product
of the current by the difference of potential between the points.
These two quantities may therefore vary reciprocally and the power
remain constant. This principle is of the utmost importance in
the electrical transmission of power. When a current flows through
a conductor, a portion of the power is spent in heating the con-
ductor, the power so spent being PR (Par. 494), or varying as the
square of the current. To avoid this waste, the current should be
kept as small as possible. From what has been shown above, we
can reduce the current and still transmit the same power pro-
vided the voltage is varied inversely with the current. An ex-
ample will bring this out more clearly.

Suppose an electric generator operated by a water wheel is pro-
ducing ten amperes at a pressure of two hundred volts, or develop-
ing a power of 2000 watts, and is transmitting power over a No. 3,
B. and S., copper wire to a factory at a distance of five miles. For
round numbers, the resistance of this wire may be taken as one
ohm per mile. The PR loss due to the resistance of the wire is
100 X 10 = 1000 watts, that is, fifty per cent of the power generated
is lost in the wire. If this same generator turned out one ampere
at a pressure of 2000 volts, it would still develop the same power,
2000 watts, but in this case the I 2 R loss would be only 10 watts,
or only one-half of one per cent of the total power. Furthermore,
if the fifty per cent loss be permissible, a No. 15 wire may be used
with the 2000 volt current and the loss still be kept within the
limit. Since the No. 15 wire weighs 52 pounds per mile as com-



396 ELEMENTS OF ELECTRICITY.

pared to 838 pounds for the No. 3 wire, there would result a saving
of 7860 pounds of copper costing about $1000.

The secret of electrical transmission of power to a distance is
therefore the employment of high potential currents. As will be
shown in Part V, high potential alternating currents are much
more easily generated and transformed up and down 'than are
corresponding direct currents, for which reasons, in the transmis-
sion of power to a distance, alternating currents are used almost
exclusively. Voltages as high as 20,000 and 30,000 are frequently
employed, and in a few cases 150,000 has been reached and power
has been transmitted upwards of three hundred miles. With
these very high voltages, the difficulty of obtaining proper insu-
lation for the line increases greatly. The wires must be spaced
widely apart on the cross arms of the poles and special forms of
porcelain insulators must be used. In rainy weather, the loss
from leakage becomes excessive. Finally, the element of danger
to life assumes serious proportions.



ELECTRO-MAGNETICS. 397



CHAPTER 37.

ELECTRIC LIGHTING.

503. The Electric Light. In Chapter 35 we examined the heat-
ing effect of the electric current. If a body be raised to a suf-
ficiently high temperature it will emit light. The production of
light by electricity is therefore only a particular case of heating.

There are at present three distinct classes of electric lights.
These are:

(a) The incandescent lamp. The current is passed through a
conducting solid which is raised to incandescence. No combustion
takes place.

(b) The arc lamp. The current is passed across the gap be-
tween two electrodes whose tips are thereby heated to incandes-
cence. A portion of one of the electrodes is volatilized and the
resulting vapor conducts the current across the gap. Combustion
takes place, but simply because air cannot be excluded.

(c) The luminous vapor lamp. The current passed through
rarefied gases or vapors contained in glass tubes causes these
vapors to glow. No combustion takes place.

504. The Incandescent Lamp. The incandescent lamp does
not differ in principle from the fuze described in Par. 483. The
earlier forms consisted of a bare platinum wire which was made
white-hot by the passage of the current. These failed because the
platinum was necessarily near its melting point and a slight in-
crease in the current would cause it to give way; moreover,
the cost of the platinum was excessive, and for these reasons the
incandescent lamp did not become a commercial success until the
development by Edison of the carbon filament. Carbon is infus-
ible and, although a conductor, is a poor enough conductor to
permit the filaments to be made of sufficient size for strength and
yet preserve the resistance required for the development of the
heating effect. If, however, carbon be heated in the presence of
oxygen it is soon consumed. The filaments must therefore be
enclosed in a vacuous glass bulb.



398 ELEMENTS OF ELECTRICITY.

505. The Carbon Filament. The first successful carbon fila-
ments were made from bamboo. Later on, they were made from
a compact paper which was cut into thread-like strips. They have
also been made from cotton thread. They are now manufactured
from a pure cotton fibre which is dissolved into a glue-like liquid
by a solution of zinc chloride. This is forced through small holes
in a die and emerges in rather soft endless threads, a little over one-
fiftieth of an inch in diameter, which are caught in a vessel con-
taining alcohol. The alcohol dehydrates and hardens the threads,
which are then washed free of the zinc chloride, coiled up and
dried. They now resemble fiddle strings. These are cut up into
the proper length, given the required shape by being wrapped
upon a form and are then embedded in pulverized carbon in a
covered crucible and carbonized at a high temperature. After
cooling, they are attached to holders, placed in a vessel in which
they are surrounded by the vapor of gasoline, and heated white
hot by a current. This process is called "flashing." The gasoline
is decomposed and deposits a semi-metallic film of gas coke on the
filaments. This renders them stronger, more uniform in resistance,
and of a steely black color. The diameter has now shrunk to .0035
inch.

An additional process recently introduced, consists in placing
the filaments, both before and after flashing, in an electric furnace
and raising them to a still higher temperature by which they are
partially graphitized. Filaments so treated are said to be "metal-
lized," and their light-giving efficiency is much increased.

506. Manufacture of the Lamps. The current enters and leaves
the glass bulb through two wires fused into a small glass tube or
stem which is inserted into the bulb and fused to it. The portion
of these "leading-in wires" which passes through the glass of the
stem (A and B, Fig. 243) must be of platinum. The coefficients
of expansion of glass and of platinum are about the same and they
therefore expand and contract together. With other metals, the
glass would either be cracked by the greater expansion of the wire
or the vacuum would be destroyed by the shrinking of the metal
away from the glass. Copper wires are fastened to the outer ends
of A and B and the filament is attached to the other ends by means
of a carbon paste. One of the copper wires is soldered to the brass
shell which carries the screw threads of the lamp base. The bot-
tom of this shell is closed by a glass or porcelain button in the center



ELECTRO-MAGNETICS. 399

of which is a brass contact, pierced with a small hole. The remain-
ing copper wire is drawn through this hole and soldered to the
contact. The shell is fastened to the bulb by a cement or by plaster
of Paris. Lamp sockets are so arranged that when a bulb is
screwed in, the required connections are made.

A small tube is left at E. This is now attached to an air pump
and most of the air is withdrawn from the bulb. When a good
vacuum has been obtained, a current is sent through the lamp.
This drives out the gases which have been occluded in the carbon
filament. The last traces of oxygen are removed by igniting a
small amount of phosphorus inserted for that purpose at E, and
E is then sealed by a blow-pipe flame.




Fig. 243.

In lamps with long and slender filaments, the filaments are liable
to be broken by excessive vibration, or when hot may droop, touch
the bulb and crack it. To remedy this they are often supported
at their middle point by a short wire, one end of which is fused into
the tip of the glass stem on the interior of the lamp. Such fila-
ments are said to be "anchored."

Incandescent lamps are run at constant voltages. Since the
heating effect, on which the light-giving effect depends, varies as
PR = IE (Par. 477), and since E is constant, the lighting effect is
increased by increasing the current. This is done by decreasing
the resistance of the lamp, that is, by making the filament shorter
and stouter.

507. Recent Incandescent Lamps. As has just been stated,
the light-giving effect of an incandescent lamp increases with the
temperature. It is therefore desirable to heat the filament as
highly as possible. As the temperature of the ordinary carbon
filament increases, so does the brilliancy of the light it emits, but
the life of the lamp is very much shortened thereby and it is not
found practicable to exceed a temperature of 1350 C.



400



ELEMENTS OF ELECTRICITY.



We saw (Par. 504) that in the early lamps attempts were made
to use platinum filaments. Platinum, which fuses at 1775 C,
was the most infusible metal which could then be obtained yet
had to be abandoned because the filaments melted. There are
known, however, certain rare metals whose fusing points are much
higher than that of platinum. Among these are osmium, tantalum
and tungsten, this last fusing at 3200 C. Their rarity, the dif-
ficulties of their metallurgy, and their consequent cost prohibited
their use. These metals may now be obtained and are success-
fully used in incandescent lamps. Their conductivity being so
much greater than that of carbon, in order to secure the necessary
resistance they must be drawn into extremely fine wires. When
they have been drawn down so that they look almost as slender as
a spider's web, their resistance is still too small and can be in-




Fig. 244.

creased only by taking longer portions for filaments, about twenty
inches on an average. Even with this length, it is stated that as
many as 20,000 may be made from a single pound of tantalum, and
this although the specific gravity of tantalum is greater than that
of lead. To insert these long filaments into the lamp bulb, they
must be folded back and forth a number of times and having very
little rigidity when cold and becoming soft when heated, they must
be supported at several points. The expansion and contraction of
a twenty-inch filament, especially if it be attached to supports at
intermediate points, is very liable to break it, for which reason it
is found better to cut the filament into four or five pieces and to
connect these pieces in series. Even in this case, especial provi-
sion must be made to allow for the expansion and contraction.
Fig. 244 shows diagrammatically the arrangement of the filament
in a tungsten lamp. The leading-in wires pass through a glass
stem just as in the carbon lamp. To this stem and in prolongation
of it there is fused a slender glass rod expanded into a button at



ELECTRO-MAGNETICS. 401

A and at B, points about two inches apart. Into the button A
there are fused four V-shaped pieces of wire, the vertices of the V's
being embedded in the glass so that the free ends radiate like the
spokes of a wheel. Into the button B there are fused five equi-
distant straight pieces of wire. These are shorter than the pieces
in A, but are brought out to the same length by an attached piece
of the filament wire, this last terminating in a small circular loop.
A piece of the filament is attached to the terminal C, the free end
is then threaded through the loop D and brought back and at-
tached to E. A second piece is attached to F, carried through the
loop G and fastened to H, and so on around the axis. A develop-
ment of these connections is shown at the right of Fig. 244, whence
it is seen that the successive pieces of filament are in series. The
flexible ends of those arms which radiate from B allow for the ex-
pansion and contraction of the filaments which they support.
These lamps produce a very fine white light with a smaller expendi-
ture of energy than in the case of the carbon lamp.

508. The Nernst Lamp. The oxides of certain of the rarer
metals, yttrium, thorium, zirconium, are infusible and if highly
heated emit a very bright light. It is on this account that these
oxides are used in the mantles of the Welsbach burner. When
cold, they do not conduct electricity but if heated to about 700 C
they become conductors and if a current be now passed through
them they may be heated to a point where they glow with great
brilliancy. This property is utilized in the Nernst lamp. The
light is emitted from a glower, a little rod of these oxides about
two centimeters (three-quarters of an inch) long and one millime-
ter in diameter. The light-giving power of a lamp is increased by
using more than one glower. The lamp must be provided with
an auxiliary arrangement by which (a) the glower is heated up to
the conducting point and (b) the current is then switched from the
heater to the glower.

Fig. 245 shows diagrammatically the operation of the lamp.
A is an armature, bent at an angle and pivoted as shown. Its
shape causes its lower end to hang out and make contact at C.
H is the heater, a slender porcelain tube around which is wrapped
a coil of very fine platinum wire which, for protection, is embedded
in a white cement paste. M is an electro-magnet with an L-shaped
core. The current enters at D, travels down A, passes through the
contact C, around H and out by E. The passage of the current



402



ELEMENTS OF ELECTRICITY.



through H heats it and in less than half a minute the glower G has
been raised to a conducting temperature. The current entering at
D may now pass around M, through the resistance B, through G
and out. M becomes magnetized, the armature A is attracted and
the contact at C is broken. The full current now passes through (7.
Owing to the method of operation of the current shifter, these
lamps are restricted to a vertical position.




Fig. 245.

As the temperature of G rises, its resistance decreases. This
would permit a larger current to flow through G and its tempera-
ture would rise still higher, and so on, until the glower would be
melted. This rise of current, however, is controlled by the resist-
ance B, a fine iron wire which, to prevent oxidation, is sealed up in
a glass tube in an atmosphere of nitrogen. It is adjusted to permit
the passage of the required current at the voltage for which the
lamp is intended. The resistance of iron increases rapidly with
the temperature and an increase of seven per cent in the current
will double the resistance of B. Variations in the voltage do not,
therefore, cause proportional variations in the current through the
lamp. A resistance such as B, which steadies or prevents undue
fluctuations in the current, is commonly called a "ballasting coil"
or simply "ballast."

Since the glower is composed of oxides, it is not necessary to
seal it up in a bulb. It is, however, usually surrounded by a
glass globe. Doubtless on account of electrolytic action, the



ELECTRO-MAGNETICS. 403

life of a glower is less with direct current than with alternating
current.

509. Candle-Power. Lamps are rated according to the in-
tensity of the light which they emit under normal conditions, as
4, 8, 10, 16, 32, 50, and 100 candle-power. The British standard
candle is defined as a spermaceti candle, seven-eighths of an inch
in diameter, weighing one-sixth of a pound, and burning at the
rate of 120 grains per hour. The German standard, the Hefner
unit, or the Hefner, is the light emitted by a lamp of prescribed
dimensions burning amyl acetate. The hefner is about .88 of a
candle-power. In actual measurements of candle-power, use is
made of secondary standards, incandescent lamps whose candle-
power has been determined by comparison with the primary units.
The standards in use in this country are determined from the
hefner.

In many electric lamps, the light emitted in certain directions
is greater than that emitted in others. Such lamps are frequently
rated according to their mean spherical candle-power, that is, the



Online LibraryWirt RobinsonThe elements of electricity → online text (page 31 of 46)