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EXPERIMENTAL PHYSIOLOGY. 



Fig. 42. Spring Key. 



8. The Contact or Spring Key (Fig. 42) is also very useful 



for rapidly making and breaking a 

 circuit. The current can only pass 

 between the binding screws when the 

 metallic spring is pressed down. The 

 left end of the spring is in metallic 

 contact with the upper binding screw, 

 while the second binding screw is 

 similarly connected with the little 

 metallic peg at the right-hand end of the fig. 



9. Means of Graduating a Galvanic Current . Besides altering 

 the number, arrangement, or size of the cells themselves, we 

 can use an arrangement to divide the current itself, the battery 

 remaining constant. This is effected by the simple rheocord. 



The Simple Rheochord consists of a brass or German-silver wire 



about 1 metre in 

 length, stretched 

 longitudinally along 

 a board, and with its 

 ends connected to 

 binding screws and 

 insulated (Fig. 43). 

 On the wire there is 

 a " slider " which can 

 be pushed along as 

 desired. Apparatus. 

 Simple rheochord, 

 Daniell's cell, detec- 

 tor, du Bois key, five 

 wires. 



(a.) Arrange the experiment as in Fig. 43. When the 

 slider S is hard up to W, practically all the electricity passes 

 along the wire (W, R), back to the battery. 



(6.) Pull the slider away from W, and in doing so, more 

 resistance is thrown into the battery circuit, and some of the 

 electricity passes along the detector circuit and deflects the 

 needle. The deflection is greater but not proportionally 

 so the further the slider is moved from W. 



10. Use in the same way a rheochord with two platinum wires, 

 which are connected by an ebonite cup filled with mercury, which 



Fig. 43. Scheme of Simple Rheochord. B, Bat- 

 tery ; K, key ; W, R, wire ; S, slider ; D, 

 detector. 



