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



the primary circuit, the second was connected with the thermo- 

 meter, and the third was closed by wires of successively increa- 

 sins; retarding; values. The results obtained are as follows : — 



When the third spiral was open, the value of the secondary 

 current induced in the second was 100 ; the uniting of the ends 

 of the third spiral by a copper wire 23 inches in length brought 

 the secondary current down to 61. Now it is proved by expe- 

 riment, that the action of a secondary spiral upon the primary 

 current when its ends are connected by a wire of the above 

 length (.23 inches), is just the same as if the spiral were left open ; 

 hence the diminution of the secondary current is due, not to any 

 modification which the primary has undergone, but to the direct 

 action of the other secondary. The first four of these experi- 

 ments show, moreover, that the stronger the current in the third 

 spiral, the greater is the amount of weakening in the second*. 

 By increasing the resistance in the third spiral, the primary cur- 

 rent at length becomes modified, and a decrease of the current 

 in the second spiral up to a certain point is the consequence ; 

 from this point forward the current again increases, until with 

 a retarding value of 12G459 it attains almost the same strength 

 which it possessed when the third spiral was altogether inactive. 

 The result of this immediate and mediate action of the one secon- 

 dary upon the other, is the occurrence of two maxima and two 

 minima in the series of observations. 



* Hence the formation of a secondary current is checked by permitting 

 the primary wire to excite a second secondary at the same time j the 

 reader will do well to remember this, as the circumstance is turned to 

 account further on. 



