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we are able to determine how much heat is produced by the action of 24, 53, 82, 101 coils of the main wire. This gives

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Thus the heat produced is very nearly proportional to the number of acting coils of the main wire; hence it follows that the quantity of electricity generated by the conducting circuit of the battery in a secondary wire is proportional to the length of the acting part of the circuit wire, other circumstances being equal.

If over the same main spiral A A the same lateral spiral be wound, first with its coils parallel to those of the main spiral, and then with more open coils, so that the main spiral acts always in the direction. of its entire length, but at first upon a long part of the lateral wire running parallel with it, and then on a shorter and more open part; in the latter case the action evidently is as much less as the direction of the coils in the spirals differs, or the closer the lateral spiral is in comparison with the main spiral.

All the coils used for these experiments were wound to the right. It is not a matter of indifference, as far as regards the strength of the secondary current, whether the lateral spiral is wound in the same or the opposite direction to that of the main spiral. Upon a main spiral wound to the right, eight inches of copper wire were wound first to the right, then to the left, with the result:

Secondary spiral to the right......

66 to the left...

Heat.

15.4

2.7

§ 60. Action of the main wire on different secondary wires.-A piece, (a b,) 26 inches long, of the same wire which formed the main wire was stretched out straight; parallel with it a piece (c d) of the lateral wire was stretched. The whole secondary circuit, in which the electrical thermometer was inserted, consisted of copper and iron wire. The piece cd of the secondary circuit, lying opposite a b, being a part of the iron or of the copper wire which forms the lateral circuit, with equal charges of the battery the temperature of the thermometer is the same, provided the iron and copper wire have the same diameter and the space between a b and c d is the same.

Therefore, if the resistance to conduction of the whole secondary circuit remains unchanged, it is perfectly indifferent for the strength of the secondary current whether a better or worse conducting piece of wire is exposed to the action of the main wire.

It is impossible for me to understand clearly the arrangement of the experiments relating to this matter from the description given.— (Pog. Ann., L, 3.)

§ 61. Decrease of the secondary current in proportion to the distance from the main wire.-To find how the action on the secondary wire decreases with the distance from the main wire, the piece running parallel must have a great length, because otherwise, at tolerably

great distances, the heating of the lateral wire will be too little to be observed.

Riess stretched two copper wires 10 feet 6 inches long parallel to each other, (Pog. Ann., L, 7.) One of them was connected by means of copper wires 6 feet long with the circuit of the battery; the ends of the other were connected by similar wires with the platinum wire of the thermometer. By changing the distance between the axes of the parallel wires the thermometer showed that the current generated by the straight part of the conducting circuit of the battery in the parallel wire decreases in the proportion in which the distance of the axis of the wires increases, provided the distance of the wires at the start is not too small; for if the wires approach within a certain limit the heat produced increases in a less proportion than the distances decrease.

To obtain somewhat elevated temperatures by the secondary current, wires of great length must be used, and the management of these is very troublesome when they have to be stretched straight. Hence, when only the generation of an intense secondary current is desired, it is greatly preferable to wind the wires in a flat spiral, as already described, (144.)

The current which is excited by the main spiral in the secondary, is weaker the further the spirals are apart; but it is easily seen that between the strength of the current and the distance between the spirals there cannot be a simple proportion, for any one part of the circuit of the main spiral excites a current, not only in the curved part lying nearest to it and on the same side, but also in the more remote part of the curve, on the opposite side; the latter is indeed weaker, but it acts against the former and diminishes its effect. But the proportion of the two opposite currents evidently changes when the distance of the spirals is changed. If the starting point is from very small distances of the two spirals the strength of the secondary current at first increases more slowly, but at a greater distance far more rapidly than the increase of the distance of the spirals.

§ 62. Action of adjoining closed conductors on the generation of the secondary current.-Riess extended on the floor of a room three copper wires, 0.55 line thick and 10 ft. 6 in. long, parallel to each other, (Pog Ann., L, 12,) these wires being denoted respectively by A, B, and C. The axial distance between A and B was 4.45 lines, that of B and C 2.35 lines.

The wire A was inserted in the conducting circuit of a battery; from the ends of the wire C copper wires six feet long led to the thermometer, and consequently the secondary wire C included the thermometer in its circuit. When B was removed the unit of charge gave a temperature indication of 0.135; B being restored to its place nearly the same temperature was indicated; but when the ends of B were joined by a copper wire 14 feet long only 0.094 was the temperature indicated. Hence it follows that

The current generated in a secondary wire by the conducting wire of a battery remains unchanged when a wire with free ends lies between the two wires; but the current is diminished if the intermediate wire is closed upon itself.

It is not essential that the wire B should lie between A and C in order to weaken the current in C, which is generated by the discharge current traversing A. B may lie beyond C or beyond A; the lateral current excited in C by the main current of A will be always weaker when B is closed, or when a secondary current exists in B, than when this is not the case. Hence, the main wire of a battery having generated electrical currents in two secondary wires near each other, each of the two secondary currents is weaker than it would have been were the other not present.

Two flat spirals, six inches in diameter, each formed of copper wire 13 feet long and 0.55 lines thick, were placed 10 lines apart. The thermometer of the secondary spiral indicated a considerable heat (42 division of the scale) when the quantity of electricity (20) accumulated in four jars was discharged through the main spiral. But when, under otherwise equal circumstances, the same quantity of electricity was discharged, while a copper disk 6 inches 10 lines in diameter and 0.33 lines thick was interposed between the spirals, the thermometer of the secondary spiral showed no sensible heat.

This remarkable effect of the copper plate evidently depends upon the good conduction which it offers to the current.

The interposed plate should be a poor conductor to allow a sensible heat to be developed in the secondary spiral. In proportion as the capacity for conduction in the interposed plate decreases the current in the secondary spiral increases.

Interposing plates were used successively as follows: 1. A sheet of tin foil 0.01 line thick. 2. One of 0.0168 line thick. 3. Both together. 4. A sheet of imitation silver paper. These sheets were clamped between glass plates and placed one line distant from the main spiral. When the two spirals were two and a half lines apart the following temperatures were obtained in the secondary spiral for the unit of charge:

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Comparing the last three indications with the corresponding thicknesses of the interposed sheets of tin foil, we find that the strength of the current in the secondary wire is inversely proportional to the thickness of the interposed metallic plate.

The same result was obtained by repeating the experiments in the same manner but at greater distances.

§ 63. Action of interposed insulating plates upon the formation of the secondary current.-Faraday has ascribed a specific inductive capacity to the different insulators in relation to statical electricity, so that through a glass or shellac plate induction should be much stronger than through air.

The origin of the secondary current can only be satisfactorily explained by the generation of electricity by induction; and, in his view, we should expect currents of different strengths, if plates of different

insulating substances were interposed between the main and secondary spirals.

If solid insulators possess a greater specific inductive capacity than air a well marked distinction should be made by means of the secondary current between solid conductors and insulators of electricity. Thus, while conductors, used as interposed plates, diminish the secondary current obtained through the medium of the air, insulators, applied as interposed plates, should increase the current.

In spite of careful investigation Riess was unable to find such an increase of the secondary current by the interposition of insulating plates, such as glass, shellac, &c. The use of these plates changes in no respect the force of the secondary current, which was found just as great as though air only had been between the spirals.—(Pog. Ann., L, 18.)

§ 64. Action of the conducting wire of a battery upon itself.-We have seen that no electrical current can be generated by induction in a wire with free ends. The conducting wire of an electrical battery is such a wire, but since its free ends pass into broad metallic surfaces, allowing the accumulation of opposite electricities, it is necessary to examine experimentally whether one part of the wire may not have an inductive action on another part.

Riess sought to solve this question in the following manner: (Pog. Ann., L, 19.)

The two spirals, one of which had served hitherto as the main, the other as the secondary spiral, were placed at a short distance apart, and joined so as to form a single conducting wire, so that, on being introduced into the circuit of the battery, the discharge current had to pass through both.

In one case the outer end of one of the spirals was united with the central end of the other in such a way that when the discharge current in the one spiral passed from the middle to the outside, it had to pass from the middle to the outside in the other also; and, consequently, the discharge traversed the two spirals in the same direction.

The outer end of one spiral was then joined to the outer end of the other, so that the current which traversed the one from the middle to the outside went from the outside to the middle in the other; the discharge thus traversing the two spirals in opposite directions.

Now, if one part of the conducting circuit can act upon another, each spiral in the first case must cause in the other a current in the same direction as the main current, but in the last mode of connecting the spirals a current opposed to the main current; and hence, in the last case the force of the current, cæteris paribus, should be weaker than in the first.

The thermometer being introduced into the circuit along with the combined spirals, it indicated, under like circumstances, perfectly equal temperature, in whichever manner the spirals were united; hence it follows, that in the discharge of a battery no part of the conducting wire acts inductively upon another part.

§ 65. Retardation of the electrical discharge by conductors near the conducting wire of a battery.-Riess introduced into the conducting

circuit of a battery (Pog. Ann., XLIX, 393) a copper wire 13 feet long and 0.55 line thick, which was coiled in a flat spiral on a wooden disk six inches in diameter, covered with pitch and supported by a glass leg, as represented by fig. 61. A series of experiments, made with the circuit thus arranged, gave

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A copper plate 6 inches 10 lines in diameter and 0.33 line thick was placed parallel to the main spiral, at a distance of 2 lines. It gave

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Then a secondary spiral exactly like the main spiral was placed parallel to it, the ends being in perfect metallic contact. This arrangement gave

h = 0.42

0.42 2o

Hence, neither the copper disk nor the secondary spiral had a sensible influence on the temperature of the conducting circuit. Instead of the perfect metallic closure, a less perfect closure of the secondary spiral was made; that is, the ends of the copper wire were connected by a platinum wire 138 lines long and 0.023 in. radius. The secondary spiral thus closed being placed 5 lines distant from the main spiral the result was

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when placed at the distance of only 2 lines from the main spiral the result was

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The secondary spiral, closed by a German silver wire 460 lines long and one-twelfth line diameter, and placed 2 lines from the main spiral, gave

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The secondary spiral, closed by a glass tube filled with water 9 inches long, gave

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We will now subject these results to a somewhat closer examination. The current in the conducting circuit, as seen above, generates a

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