10 PEACTICAL PHYSIOLOGY 



movable horizontally on a central point, the strength of the induced 

 current can be graduated by altering the angle between the two coils. 

 (4) It varies inversely as the distance between the two coils, being 

 greatest when the secondary is completely over the primary coil, and 

 becomes less and less as the coils are separated. The strength of the 

 induced current is usually regulated by varying the linear distance 

 between the coils, and most induction-coils are graduated by a milli- 

 metre scale fastened to the side of the carrier, so that the pointer in 

 the secondary coil is at the zero of the scale when the one coil is exactly 

 covered by the other. This graduation, however, is purely arbitrary, 

 for the absolute decrease in the strength of the induced current becomes 

 less and less for every centimetre that the coils are separated. An 

 exact graduation can be obtained by a scale corresponding to equal 

 galvanometric deflections. 



The direction of the induced current in the secondary coil is, at make 

 of the battery-circuit, in the opposite direction, and at break of the 

 battery-circuit, in the same direction as the battery-circuit in the 

 primary coil. Most coils are so wound that when at make the battery 

 current enters the primary coil by one top binding-screw, the induced 

 current leaves the secondary coil by the binding-screw of the opposite 

 side (Fig. 16). 



The Use of Make- and Break-Induction Shocks as Stimuli. Two 

 wires are connected with the poles of a Daniell cell ; the free end 

 of one wire is fastened to one binding-screw of a spring-key, and to 

 the other screw of the key is fixed a third wire. The clean free ends 

 of the wires are placed on the tongue, and the key is opened and closed ; 

 no shock is produced, but only a sensation of taste ; the intensity of the 

 current is insufficient to produce a marked excitation. 



The free ends of the wires are now connected with the screws, or 

 terminals, 1 and 2 of the induction-coil and a Du Bois key is placed 

 in the secondary circuit (Fig. 16). The secondary coil is pushed far 

 apart from the primary, and the Du Bois key is opened ; make and 

 break of the primary circuit produces no excitation, for the induction- 

 currents are too weak. The secondary coil is gradually moved 

 towards the primary, and the spring-key is opened and closed from 

 time to time, until a point is reached at which a shock is felt at 

 break, but not at make of the constant current. The position of the 

 secondary coil on the scale is noted. As the secondary coil is moved 

 up further, the break-shock becomes greater, and a slight shock is also 

 felt at make ; in a similar way the two shocks can be further increased, 

 but the break-shock remains greater than the make-shock. 



It is especially to be noted that there is no induction-shock if the 



