ELEMENTARY EXPERIMENTAL PHYSIOLOGY 9 



fine copper wire. The large number of turns of wire in the secondary 

 as compared with the primary coil, transforms the low E.M.F. of the 

 current in the primary circuit into a high E.M.F. in the secondary 

 circuit; for each turn in the primary coil induces an effect in every 

 turn of the secondary coil, so that the sum of all these effects is a single 

 one of greatly increased intensity. 



The long fine wire of the secondary coil gives it a great resistance, 

 but when the induced currents are passed through the relatively enor- 

 mous resistances of animal tissues this is unimportant. 1 



The ends of the wire of the secondary coil are connected to the 

 binding-screws 3 and 4 (Fig. 16). 



The E.M.F. of the induced current varies with the following factors : 

 (1) It varies directly with the intensity of the change of current in the 

 primary circuit, so that if no current or a current of constant strength 

 be running through the primary coil no induction occurs ; but if the 

 strength of the current in the primary circuit does change, whether it 

 be an increase or decrease, the greater the change the stronger will be 

 the induction. (2) It varies directly as the rate of change in the 



s.c - 



FIG. 16. Diagram of an induction-coil and its connections. 



strength of the inducing current, so that, if the constant current be 

 increased or decreased greatly in strength, but sufficiently gradually, no 

 induction takes place; on the other hand, for a given change in the 

 constant current the more rapid the change the greater the induction. 

 (3) It varies with the angle between the primary and secondary coils 

 in such a way that when the two coils are accurately at right angles 

 there is no induced current ; but the strength of the induction increases 

 as the angle between the coils is altered until the maximum is reached, 

 when the wires are parallel to each other. If the secondary coil be 



1 The resistance of a piece of a frog's sciatic nerve 1 cm. long is about 

 100,000 ohms. 



