6 



EXPERIMENTAL PHYSIOLOGY 



each tarn of^the primary induce currents in each neighbouring turn 

 of the coil. If we consider two neighbouring turns when the current 

 is suddenly increased, the increase in the one wire will induce a 

 current in the second, and this induced current will be in the reverse 

 direction to that of the main current, and as the direction of the 

 current in two neighbouring turns is the same it tends to diminish 

 the amount of the increase in the second wire. As the duration of 

 this induced current is very short its effect is soon exhausted, but not 

 before it has produced the result that more time is required for the 

 current to reach its full strength than would have been the case if the 

 wire had been perfectly straight. On breaking the circuit the circuit 

 of the primary is broken, so that no induction currents can be set up 

 in the primary. The fall in potential is therefore instantaneous. 

 These effects are diagrammatically represented in fig. 7. In this 



figure, lines written hori- 

 ^AffE break zontally indicate time, and 



vertical lines strength of 

 current. At the instant a 

 a current whose amount is 

 represented by the vertical 

 line a c is suddenly thrown 

 into the primary, but in- 

 stead of instantly reaching 

 its full intensity, when the 

 course of events w T ould be 

 represented by the line A c, 

 time is occupied before it 

 attains its full strength. Thus the gradual rise of strength of the 

 current is represented by the curved line a b. At the instant G 

 the current is broken, and there occurs an instantaneous fall in 

 strength to zero, which is thus represented by the line F G. The 

 induction effect produced in the primary on making the circuit is 

 spoken of as the ?nake extra-current. The result of this upon the 

 current induced in the secondary coil is of very great importance. 

 One of the chief factors varying the intensity of the induced current 

 is the rate at which the change is effected, and as the make takes an 

 appreciable time while the break is instantaneous, it follows that the 

 induced secondary current at make is of less E.M.F. than that at break, 

 but lasts longer. This is indicated in the lower half of fig. 7. The 

 line k r indicates zero current, and the curved line klm the current 

 induced in the secondary by the change of current a b in the primary. 

 The intensity of the change at any instant is indicated by the vertical 

 height of the curve for that instant, and is drawn below the line K M, 



PR/MARY 



SECONDARY 



