ELECTRICAL KEYS, RHEOCHORD. 



133 



into a more or less horizontal position, but the angle of 

 deflection is not directly proportional to the current passing 

 in the instrument. Break the circuit by removing one 

 wire, and notice that the needle travels to zero and resumes 

 its vertical position. The detector made by Stohrer, of 

 Leipzig, is a convenient form. 



LESSON XXIII. 

 ELECTRICAL KEYS, RHEOCHORD. 



It is convenient to make or break i.e., close or open a cur- 

 rent by means of keys of which there are various forms. 



1. Du Bois Key (Fig. 37). It consists of a square plate of 

 vulcanite, attached to a wooden or metallic framework which 



can be screwed to a table. Two 

 oblong brass bars (II. and III.), each 

 provided with two binding screws, are 

 fixed to the ebonite, while a movable 

 brass bar (IV.) with an ebonite 

 handle is fixed to one of the bars, and 

 can be depressed so as to touch the 

 other brass bar. 



Two Ways of Using the du Bois Key. 

 2. (1) When the key is closed the 



Fig. 37. Du Bois - Raymond's 

 Friction Key. 



Fig. 38. Scheme of du Bois Key. B, 

 battery; K, key; N, nerve; M, 

 muscle. 



