Otherwise in a simple closed circuit of two metals if one junction were heated a little to begin with, a current would be set up which would still further increase the difference in temperature of the junctions and would so become continually stronger and might be used to run a motor and do mechanical work until all the heat energy in the thermopile was used up and it was reduced to the absolute zero of temperature.

671. Thomson Effect. In 1854, Lord Kelvin (Sir William Thomson) showed that in a thermoelectric circuit there must in general be electromotive forces not only at the junctions, but also

Lord Kelvin as a consequence of the law of energy and was then verified by the following experiment.

A bar of iron was set up as shown in figure 381 so that the center was heated by boiling water while the ends were cooled with ice. When a current was established all parts of the bar were warmed, but a thermometer at A was observed to stand higher when the current was from left to right than when it was reversed, while the opposite was true at B.

672. Applications.-The thermopile as a delicate means of observing the intensity of heat radiation has already been described (§665).

A particularly sensitive instrument for the same purpose was devised by Boys and is known as the radio-micrometer. In this instrument a simple circuit of bismuth and antimony is suspended between the poles of a powerful magnet by a fine quartz fiber. One of the two junctions is protected from outside radiation by the surrounding instrument, while the other hangs in an opening so that radiation may be directed upon it. The slightest difference of temperature causes an electromotive force and since the resistance of so short a circuit is very small a comparatively large current is produced, which, reacting on the magnetic field, causes the suspended circuit to turn. A light mirror mounted on the suspended system turns with it so that the angular deflection may be read by a telescope and scale.

For the measurement of high temperatures a thermal couple consisting of a wire of pure platinum joined to another of an alloy of platinum and rhodium may be used. In the Le Chatelier pyrometer such a couple,

mounted in a protecting sheath of porcelain, is thrust into the furnace or oven of which the temperature is to be determined; wires from the couple lead to a suitable galvanometer graduated to read temperatures directly up to 1500° C.

For the measurement of ordinary temperatures a thermal couple of iron and German silver is often convenient.

Problems.

1. Find the thermal electromotive force of an iron-copper circuit in which one junction is at 0° and the other at 200° C.

2. Find the increase in electromotive force in a lead-iron circuit when the temperature of the hot junction is changed from 150° to 151°. 3. What relation must the lines of two metals on the thermo-electric diagram bear to each other in order that the increase in electromotive force per degree rise in temperature of the hot junction may be a constant?

4. When one junction of zinc-iron circuit is at 50° C., at what different temperature may the other junction be without causing any current in the circuit?

MAGNETIC EFFECTS OF Currents.

673. Oersted's Experiment. The first evidence of the magnetic action of an electric current was obtained in 1819 by the Danish physicist Oersted, who discovered that when a wire carrying a current is held in a north and south direction over or under a balanced magnetic needle the needle is deflected. as shown in figure 382; and if the directive force of the earth's magnetism is neutralized by means of a magnet, the needle sets itself at right angles to the

current.

Wire above Wire under needle needle

FIG. 382.-Oersted's Experiment.

674. Magnetic Field Around a Straight Conductor. The experiment of Oersted indicates that the magnetic force due to a current is in a plane at right angles to the current. To investigate its direction more fully cause a strong current to flow in a wire which passes vertically through a card on which some fine. iron filings are scattered; on tapping the card the filings arrange

themselves in circles about the wire as shown in figure 383. If the current is down as shown by the arrows, a small compass needle near the wire, at any point such as P, will point with its north pole in the direction of the arrow at that point, tangent to the circle. If the current is reversed the compass needle will point in the opposite direction.

The lines of magnetic force about a straight conductor carrying a current are circles of which the conductor is the axis.

By a comparison of figures 383 and 384 it will be seen that the positive direction of the lines of force bears the same relation

eceee

FIG. 383.-Field around current.

FIG. 384.-Right-handed screw.

to the direction of the current as the direction of rotation of a righthanded corkscrew bears to the direction in which it advances.

Another rule that may be given is that if an observer looks along a conductor in the positive direction of the current, the positive direction of the lines of force as he sees them is clockwise.

675. Strength of the Field. The strength of the magnetic field near a straight conductor is greatest next to the conductor and diminishes as the distance increases.

The strength of field H, at a distance r from the axis of a long straight wire carrying a current of strength I, is given by the expression

21 H

r

(all the quantities being in C. G. S. units), provided that the return circuit is so far off that its effect may be neglected.

As the card is tapped on which the iron filings rest, in the experiment described in the last article, the filings work toward the center, the circles gradually getting smaller, for the filings are drawn from a weaker field toward a stronger.

If a fine copper wire carrying a rather strong current is dipped into some fine iron filings they will cling together in little circular filaments, forming a mass around the wire.

676. Field of a Circular Current.-When a conductor carrying a current is bent into a circle the lines of force are crowded In this case, together within the circle and spread out outside. shown in figure 385, all parts of the circuit conspire to cause magnetic lines of force of

which the direction is through the circuit perpendicular to its plane on the inside, and back again on the outside, as shown in the diagram. The lines of force very near the wire are nearly circles about the wire, while at the center they are nearly straight and perpendicular to the plane of the coil.

If the wire carrying the current makes two turns around the circle instead of

FIG. 385.-Field of circular current.

one, the magnetic force will everywhere be doubled, and so on for any number of turns.

The strength of the magnetic field at the center of a circular current is proportional to the total length of the conductor wound in the circle and to the strength of the current and inversely proportional to the square of the distance of the conductor from the centre.

Thus if r is the radius of the coil and if n is the total number of turns, the length of the wire in the coil is 2arn, and the force at the center is proportional to

where I represents the current strength..

677. Rowland's Discovery.-Rowland discovered that a disc of ebonite, charged with electricity and rotating at high speed, acted upon a magnetic needle placed near it, just as a circu

lar current would. The magnetic effect was found to be proportional to the speed of the disc. This remarkable experiment was carried out by him in 1875.

678. Electromagnetic Unit Current, C. G. S. System. - In dealing with electric currents it is found convenient for scientific purposes to use a system of units based on the magnetic action. of a current, retaining the magnetic units as already defined. This system is known as the C. G. S. electromagnetic system, since it is based also on the centimeter, gram, and second.

In this system a unit current is one which, flowing in a circular coil of one centimeter radius, will act on a unit magnetic pole at its center with a force of one dyne for every centimeter of wire in the coil.

If a current I is measured in these units, the strength of magnetic field H produced at the center of a coil of radius r is given by the formula

The Practical Unit of Current or Ampère.-The ampere, or unit of current in the practical system, is based upon the C. G. S. electromagnetic unit, and is defined as one-tenth of that unit. Hence if I' is the strength of current in ampères in a circular coil of n turns, of radius r centimeters, the force at the center in dynes per unit magnet pole, is

The formula assumes that the cross section of the coil is negligibly small compared with r.

By measuring the magnetic force at the center of a coil of known dimensions, the number of ampères of current in the coil may be determined.

679. Solenoid.-A long helix, such as shown in figure 386, is known as a solenoid, and may be wound with one or several layers. When a current passes through such a coil all the turns act together to cause a field of magnetic force in which lines of force pass lengthwise through the interior looping back around the outside. Looking through the solenoid in the positive direction of the lines of force, the direction of the current is clockwise. In