178 Dr. Tyndall on the Progress of the Physical Sciences : 



correct, as the following experiments prove. It will add much 

 to the reader's comfort if a clear conception of the arrangement 

 of the spirals and thermometer be obtained. In the case now 

 to be described there were two circuits, a primary and a secon- 

 dary ; in the primary circuit were the battery and two induction- 

 discs placed one after the other, and in the secondary circuit 

 two others of the same size, and the air-thermonieter ; the two 

 primary spirals were connected by a wire which proceeded from 

 the centre of one to the rim of the other, and the two secon- 

 daries were united in the same manner ; the other ends of the 

 secondary spirals were connected with the thermometer. The 

 experiments were made in the following manner: — First, one 

 secondary was placed parallel to its primary and two lines distant 

 from it ; the battery was discharged, and the power of the secon- 

 dary current was observed on the thermometer ; secondly, the 

 other secondary spiral was brought within two lines of its pri- 

 mary, the two former being widely separated, and the heating 

 was again observed; thirdly, both secondaries were brought within 

 two lines of both primaries, and the strength of the current in- 

 duced in the whole secondaiy circuit was ascertained : the fol- 

 lowing are the results of these experiments : — 



These experiments prove that the strength of the secondaiy 

 current increases in a somewhat smaller ratio than the length of 

 the wire, a result which might be predicted when the reaction of 

 the secondaiy upon the primary cm-rent is taken into account. 

 Thus, if we imagine the secondaiy circuit divided into two 

 portions, the sum of the actions of both portions, taken sepa- 

 rately, is greater than the action of both together. 



The reaction of the secondaiy current upon the primary was 

 demonstrated by former experiments in the following manner : — 

 The primary circuit contained two induction-discs, as in the case 

 just described. Opposite to each of these was placed a secon- 

 darv disc, which could be closed so as to form a continuous cir- 

 cuit in itself; the secondary discs were first left open, and the 

 strength of the primary current was measured; the secondaiy 

 discs were next closed by short copper wires, and the primary 



