18 



primary circuit, so that the two strikers below the drum 

 make and break the circuit as the drum revolves (fig. 10). 

 (a) Always keep the key in the primary circuit open 



unless when using current. 

 (6) Always keep the key in the secondary circuit closed 



unless when stimulating, 

 (c) Keep strings clear of the switchboards. 



FlG. 10. Course of Contraction of Muscle. 

 1. Induction Coil. 2. Drum. 3. Frogboard. 4. Mercury Key. 5. Friction Key. 



(2) With the key in the secondary circuit open, with the hand 

 move the drum in the direction in which it is driven by the 

 string so as to make and break the circuit and test the 

 current passing to the electrodes by applying them to the tip 

 of the tongue. 



(a) See that wires are not broken. 



(6) See that all metallic contacts are bright and close. 



(c) See that no short circuits are present. 



(3) Arrange the driving cord of the drum to give a fairly 

 rapid speed. (Spindles Large to Middle.) 



If two students are working together it is convenient to intro- 

 duce two strikers under the drum in a straight line with one 

 another, so that each revolution of the drum will give two stimula- 

 tions of the muscle. 



B. Muscle-nerve Preparation. 



When everything is ready, and not before, make a nerve- 

 muscle preparation. 



