846 RECENT PROGRESS IN PHYSICS. 



We see here that, on prolonging the main conductor, the maximum 

 effect of the secondary wire is not reached until a greater length of wire 

 has been introduced into the secondary spiral^ and moreover that the re- 

 tarding effect of the secondary wire is now much less. During the pre- 

 vious experiments the temperature of the main wire was reduced by 

 the maximum effect of the secondary spiral to 48 per cent.; now, the 

 maximum effect of the secondary spiral produces only a reduction to 

 78 per cent, of the temperature, which would have been observed either 

 without the secondary spiral or by one perfectly closed. 



This is easy to explain. The secondary current is stronger in pro- 

 portion as the part of the main wire acting on the secondary wire is i 

 greater, and to the stronger secondary current we must also attribute 

 a greater reaction upon the discharge. The length of the main wire 

 was the same in both series of experiments, namely, 13 feet of copper ' 

 wire, which acted upon the same length of the secondary wire. In i 

 the first series these 13 feet made by far the greatest part of the circuit i 

 of the battery ; in the second a platinum wire was introduced, whose re- ' 

 tarding power was equal to a copper wire 568 feet long and 0.55 line 

 thick; consequently, in the last case, only about one-forty-fourth part 

 of the virtual length of the main wire acted upon the secondary spiral. I 



Riess caused two other spiral disks to be made, each containing 53^ 

 feet of copper wire two-thirds of a line in diameter. The large and 

 small spirals were introduced into the main circuit. 



The small main spiral being now placed opposite the small secondary 

 spiral at a distance of 2 lines, the maximum retarding action of the i 

 secondary spiral took place when it was closed with 29.6 feet of Ger- 

 man silver wire. With this maximum effect the temperature of the 

 main circuit was 76 per cent, of that which was observed without the 

 lateral spiral, or when it was perfectly closed. 



When the large secondary spiral was opposed to the large main 

 spiral at a distance of 2 lines, the maximum retarding action of the 

 secondary wire occurred when the latter was closed by 79 feet of Ger- 

 man silver wire, and in this case the temperature in the main wire was 

 reduced by the retarding action of the secondary spiral to 25 per cent. 



Finally, the two secondary spirals, properly connected, being placed 

 opposite the two main spirals, then 138 feet of German silver wire had 

 to be introduced into the secondary circuit to obtain the maximum 

 retarding effect, and the temperature in the main wire was thereby 

 reduced to 20 per cent, of that which would have been observed with- ' 

 out a lateral spiral. From these experiments it follows that — 



The maximum effect of a secondary wire upon the electrical discharge 

 attained by lengthening the secondary circuit is as much greater as the 

 length of the main ivire acting on the secondary luire is greater. But, 

 at the same time, to attain this maximum, a proportionately longer circuit 

 is required for the secondary loire. 



The length of the platinum wire in the air thermometer in these 

 experiments was 143.5 lines. This wire, which is very long in pro- 

 portion to the whole circuit, can never act inductively on the secondary 

 wire ; to make the longest possible part of the main wire act on the 

 secondary spiral, the wire in the thermometer must be shortened, by 

 which means the action of the main wire is, indeed, increased, but on 

 the other hand the sensibility of the thermometer is diminished. 



