Physics Part 45

8. State at least six conditions any one of which will put an electric bell circuit out of commission.

9. If one desires to insert a battery into a telegraph circuit already in operation, how will he determine the direction of the current in the wire?

10. If a boy who had magnetized his knife blade in a physics laboratory, pointed end south-seeking, should lose his way in the woods on a cloudy day, how could he determine his way out?

11. At a certain point the earth's field acts north, that of an electric current, east. The magnetoscope needle points exactly northeast when placed at that point. How do the two magnetic fields compare?

(2) ELECTRICAL MEASUREMENTS

=263. Galvanometers.=--In using electric currents it is often necessary or desirable to be able to know not only that a given current is weak or strong, but precisely what its strength is. We can determine the relative strengths of two currents by the use of a _galvanometer_.

[Ill.u.s.tration: FIG. 243.--The magnet is at the center of the coil.]

[Ill.u.s.tration: FIG. 244.--A moving-magnet (tangent) galvanometer.]

The older or _moving-magnet_ type of galvanometer is similar to the galvanoscope mentioned in Art. 239. It consists of a magnetic needle mounted at the center of a coil of wire. The coil is placed facing east and west, so that the needle will be held by the earth's magnetic field parallel to the plane of the coil. When a current is sent through the coil a magnetic field is produced within it. This deflects the needle, its north end turning east or west depending upon the direction of the current. (See Fig. 243.) The _coils_ of a moving-magnet or _tangent_ galvanometer (see Fig. 244) are _large_ and firmly fastened to the base, while the _magnet_ is _small_.

The _moving-coil_ type of galvanometer (see Fig. 245) consists of a _large magnet_ fastened to the frame of the device. The magnet usually has a horseshoe form to produce as strong a field as possible. The _coil_ is wound on a _light_ rectangular frame and is suspended between the two poles of the magnet. To concentrate the magnetic field, a cylinder of soft iron is usually placed within the coil. Fig. 246 represents a common form of moving-coil galvanometer.

[Ill.u.s.tration: FIG. 245.--To ill.u.s.trate the principle of the moving-coil galvanometer.]

[Ill.u.s.tration: FIG. 246.--A moving-coil (D'Arsonval) galvanometer.]

=264. Measurement of Electric Currents.=--A galvanometer enables one to _compare_ electric currents. To _measure_ electric currents it is necessary to employ a _unit_ of electrical quant.i.ty, just as in measuring the quant.i.ty of water delivered by a pipe, a unit of liquid measure is employed; thus, _e.g._, the current delivered by a given pipe may be 2 gallons of water per second, so in measuring the flow of an electric current one may speak of two _coulombs_ per second. The _coulomb_ is the unit quant.i.ty of electricity just as the unit of quant.i.ty of water is the gallon.

For most practical purposes, however, we are more interested in the _rate_ or _intensity_ of flow of current than in the actual _quant.i.ty_ delivered. The unit of rate of flow or current is called the _ampere_.

In determining the exact _quant.i.ty_ of an electric current, physicists make use of a device called a _coulomb meter_. (See Fig. 247.) This contains a solution of silver nitrate in which are placed two silver plates. The current to be measured is sent through the solution, in at one plate and out at the other. The plate where the current goes _in_, the _anode_, _A_ (Fig. 247), loses in weight since some of the silver is dissolved. The plate where the current goes _out_, the _cathode_, _C_, increases in weight since some of the silver is deposited. By an international agreement, _the intensity of the current which deposits silver at the rate of 0.001118 g. per second is 1 ampere_. This is equal to 4.025 g. per hour.

[Ill.u.s.tration: FIG. 247.--A coulomb meter, the anode _A_ is separated from the cathode _C_ by a porous cup.]

The _coulomb_ is defined as the quant.i.ty of electricity delivered by a current of one ampere during one second.

A 40-watt-incandescent lamp takes about 0.4 ampere of current. An arc lamp takes from 6 to 15 amperes. A new dry cell may send 20 amperes through a testing meter. A street car may take from 50 to 100 amperes.

=265. The Ammeter.=--The method described above is not used ordinarily for measuring current strengths on account of its inconvenience. The usual device employed is an _ammeter_. This instrument is a _moving-coil galvanometer_. It contains, wound on a light form, a coil of fine copper wire. The form is mounted on jewel bearings between the poles of a strong permanent horseshoe magnet. (See Fig. 248.) As in other moving-coil galvanometers, a soft iron cylinder within the form concentrates the field of the magnet. The form and its coil is held in balance by two spiral springs which also conduct current into and out of the coil.

Only a small part of the whole current measured, in some cases only 0.0001 pa.s.ses through the coil, the larger part of the current pa.s.sing through a metal wire or strip called a _shunt_[L] (see Fig. 248) connecting the binding posts of the instrument. A fixed fraction of the whole current flows through the coil. Its field crossing the field of the horseshoe magnet, tends to turn until its turning force is balanced by the spiral springs. As the coil turns it moves a pointer attached to it across a scale graduated to indicate the number of amperes in the whole current.

[L] A shunt is a conductor or coil connected in parallel with another conductor or circuit. It carries a part of the current.

[Ill.u.s.tration: FIG. 248.--Diagram of a commercial ammeter. _S_ is the shunt.]

It should be noted that while _all_ of the current measured pa.s.sed through the ammeter, but a small _part_ goes through the coil.

=266. Resistance of Conductors.=--With an ammeter one may study the change produced in the amount of current flowing in a wire when a change is made in the wire conducting the current. For example, if one measures with an ammeter the current flowing from a dry cell through a long and then through a short piece of fine copper wire, it will be seen that less current flows when the long piece is used. That is, the long wire seems to hinder or to _resist_ the pa.s.sing of the current more than the short piece. In other words, the long wire is said to have more _resistance_.

The resistance of a conducting body is affected by several conditions.

(a) It is _directly_ proportional to the _length_ of the conductor, one hundred feet of wire having twice the resistance of fifty feet.

(b) It is _inversely_ proportional to the _square of the diameter_; a wire 0.1 inch in diameter has four times the resistance of a wire 0.2 inch in diameter.

(c) It differs with different substances, iron having about six times as much as copper.

(d) It varies with the temperature, metals having greater resistance at a higher temperature.

Since silver is the best conductor known, the resistances of other substances are compared with it as a standard.

The ratio of the resistance of a wire of any substance as compared to the resistance of a silver wire of exactly the same diameter and length is called its _relative_ resistance.

Purified substances arranged in order of increasing resistance for the same length and sectional area (Ayrton-Mather) are given on p. 294.

Silver annealed 1.00 Copper annealed { from 1.04 { to 1.09 Aluminum annealed 1.64 Nickel annealed 4.69 Platinum annealed 6.09 Iron annealed 6.56 German Silver { from 12.80 { to 20.20 Mercury 63.30 Nichrome 67.50 Carbon { from 2700.00 { to 6700.00

=267. The ohm, the unit of resistance=, is defined by international agreement as follows: _An ohm is the resistance of a column of pure mercury, 106.3 cm. long with a cross-section of a square millimeter and at a temperature of 0C._

It should be noted that each of the four conditions affecting resistance is mentioned in the definition, viz., length, cross-section, material, and temperature. Since it is inconvenient to handle mercury, _standard resistance coils_, made of an alloy of high resistance are used in comparing and measuring resistances.

A piece of copper wire No. 22 (diameter 0.644 mm.) 60. 5 ft. long has a resistance of 1 ohm. See table p. 296.

The resistance of some telephone receivers is 75 ohms, of a telegraph sounder, 4 ohms, of a relay 200 ohms.

=268. Resistance of Circuits.=--Every part of an electrical circuit possesses resistance. In an electric-bell circuit, for instance, the wires, the bell, the push-b.u.t.ton, and the cell itself, each offers a definite resistance to the pa.s.sage of the current. The resistance _within the cell_ is termed _internal resistance_, while the resistance of the parts outside of the electric generator is called _external resistance_.

=269. Electromotive Force.=--In order to set in motion anything, some _force_ must be applied. This is as true of electricity as of solids, liquids, or gases. By a.n.a.logy that which is exerted by a battery or by a dynamo in causing current to flow is called an _electromotive force_.

The unit of electromotive force, the _volt_, may be defined as _the electromotive force that will drive a current of 1 ampere through the resistance of 1 ohm_. The electromotive force of a dry cell is about 1.5 volts, of a Daniell cell 1.08 volts. Most electric light circuits in buildings carry current at 110 or 220 volts pressure. Currents for street cars have an electromotive force of from 550 to 660 volts.

[Ill.u.s.tration: FIG. 249.--Diagram of a commercial voltmeter.]

=270. The Voltmeter.=--An instrument for measuring the electromotive force of electric currents is called a _voltmeter_ (Fig. 249). It is usually a moving-coil galvanometer, and is always of _high resistance_.

It is like an ammeter in construction and appearance. In fact, a voltmeter is an ammeter which has had its shunt removed or disconnected.

In place of a shunt, the voltmeter uses a coil of wire of high resistance (see _R_, Fig. 249) _in series_ with the galvanometer coil.

The high resistance of the voltmeter permits but a very small current to flow through it. Hence a voltmeter must be placed _across_ a circuit and not in it. In other words _a voltmeter is connected in shunt_, while _an ammeter is in series with the circuit_ as is shown in Fig.

250.

DIMENSIONS AND FUNCTIONS OF COPPER WIRES

Column headings:

B: B. & S. gauge number MM: Millimeters SA: Sectional area in square millimeters WL: Weight and length, Density = 8.9, feet per pound R: Resistance at 24C., feet per ohm C: Capacity in amperes

----+--------------+----------+---------+---------+----------+------ | Diameter | | | | | B +-------+------+ Circular | SA | WL | R | C | Mils | MM | mils | | | | ----+-------+------+----------+---------+---------+----------+------ 0000|460.000|11.684|211,600.00|107.219 | 1.56|19,929.700|312.0 000|409.640|10.405|167,805.00| 85.028 | 1.97|15,804.900|262.0 00|364.800| 9.266|133,079.40| 67.431 | 2.49|12,534.200|220.0 0|324.950| 8.254|105,592.50| 53.470 | 3.13| 9,945.300|185.0 2|257.630| 6.544| 66,373.00| 33.631 | 4.99| 6,251.400|131.0 4|204.310| 5.189| 41,742.00| 21.151 | 7.93| 3,931.600| 92.3 6|162.020| 4.115| 26,250.50| 13.301 | 12.61| 2,472.400| 65.2 8|128.490| 3.264| 16,509.00| 8.366 | 20.05| 1,555.000| 46.1 10|101.890| 2.588| 10,381.00| 5.260 | 31.38| 977.800| 32.5 12| 80.808| 2.053| 6,529.90| 3.309 | 50.69| 615.020| 23.0 14| 64.084| 1.628| 4,106.80| 2.081 | 80.59| 386.800| 16.2 16| 50.820| 1.291| 2,582.90| 1.309 | 128.14| 243.250| 11.5 18| 40.303| 1.024| 1,624.30| 0.823 | 203.76| 152.990| 8.1 20| 31.961| 0.812| 1,021.50| 0.5176 | 324.00| 96.210| 5.7 22| 25.347| 0.644| 642.70| 0.3255 | 515.15| 60.510| 4.0 24| 20.100| 0.511| 504.01| 0.2047 | 819.21| 38.050| 2.8 26| 15.940| 0.405| 254.01| 0.1288 | 1,302.61| 23.930| 2.0 28| 12.641| 0.321| 159.79| 0.08097| 2,071.22| 15.050| 1.4 30| 10.025| 0.255| 100.50| 0.05092| 3,293.97| 9.466| 1.0 32| 7.950| 0.202| 63.20| 0.03203| 5,236.66| 5.952| 0.70 34| 6.304| 0.160| 39.74| 0.02014| 8,328.30| 3.743| 0.50 36| 5.000| 0.127| 25.00| 0.01267|13,238.83| 2.355| 0.35 38| 3.965| 0.101| 15.72| 0.00797|20,854.65| 1.481| 0.25 40| 3.144| 0.080| 9.89| 0.00501|33,175.94| 0.931| 0.17 ----+-------+------+----------+---------+---------+----------+------

Important Topics

(1) _Galvanometers_: (1) moving magnet, fixed coil; (2) moving coil, fixed magnet, ammeter, voltmeter.