RECENT PROGRESS IN PHYSICS. 341 



It is not essential that the wire B should lie between A and C in 

 !»rder to weaken the current in C, which is generated by the discharge 

 :iiirrent 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 

 Vhen B is closed, or when a secondary current exists in B, than when 

 'his is not the case. Hence, ihe main wire of a hattery having generated 

 ■frical currents in iiuo secondary loires near each other, each of the tioo 



mdary currents is weaker than it luould have been were the other not 

 resent. 



Two flat spirals, six inches in diameter, each formed of copper wire 

 ;3 feet long and 0.55 lines thick, were placed 10 lines apart. The 

 hermometer of the secondary spiral indicated a considerable heat (42 

 livision of the scale) when the quantity of electricity (20) accumulated 

 n four jars was discharged through the main spiral. But when, 

 inder otherwise equal circumstances, the same quantity of electricity 

 yas discharged, while a copper disk 6 inches 10 lines in diameter and 

 ' ?>o lines thick was interposed between the spirals^ the thermometer 



the secondary spiral showed no sensible heat. 



This remarkable effect of the copper plate evidently depends upon 

 he good conduction which it offers to the current. 



The interposed plate should be a poor conductor to allow a sensible 

 : at to be developed in the secondary spiral. In proportion as the 

 anacity for conduction in the interposed plate decreases the current 

 'u the secondary spiral increases. 



Interposing plates were used successively as follows : 1. A sheet of 

 'in foil 0.01 line thick. 2. One of 0.0168 line thick. 3. Both to- 

 gether. 4. A sheet of imitation silver paper. These sheets were 



niped between glass plates and placed one line distant from the main 

 uiral. When the two spirals were two and a half lines apart the fol- 

 lowing temperatures were obtained in the secondary spiral for the unit 

 •f charge : 



Without interposed plate 0.56 



Interposed plate of imitation silver paper 0.57 



" " thin tin foil 0.087 



" '' thick " 0.056 



" " both sheets tin foil 0.034 



Comparing the last three indications with the corresponding thick- 

 lesses of the interposed sheets of tin foil, we find that the strength of 

 he current in the secondary loire is inversely proportional to the thick- 

 £88 of the interposed metallic plate. 



The same result was obtained by repeating the experiments in the 

 ame manner but at greater distances. 



§ 63. Action of interposed insidating plates upon the formation of the 

 \econdary current, — Faraday has ascribed a specific inductive capacity 

 ^o the different insulators in relation to statical electricity, so that 

 hrougli a glass or shellac plate induction should be much stronger 

 -ban through air. 



!i The origin of the secondary current can only be satisfactorily ex- 

 plained by the generation of electricity by induction ; and, in his view, 

 ve should expect currents of difierent strengths, if plates of different 



