l6o PRACTICAL PHYSIOLOGY. [XXVL 



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. 



8. Effect of Constant or Voltaic Current on the Tongue. — 



Apply the free ends of the wires to the top of the tongue and note 

 tlie effect of the current ; or a key may be placed in the circuit. 

 The physiological effects of a moderate constant current are but 

 slight on the sensory nerves of the tongue, there being perhaps a 

 slight metallic taste. 



Electrical Units are :— The unit of current is an ampere, the 

 unit of resistance an ohm, and the unit of pressure a volt. The 

 pressure or potential of a Darnell's cell is about i volt. One 

 ampere current is obtained by i volt pressure through i ohm 

 resistance, through 20 ohms ^V ampere. The internal resistance 

 of an ordinary cell varies from i to 10 ohms. 



LESSON XXVL 



ELECTRICAL KEYS— RHEOCHORD. 



It is convenient to make or break — i.e., close or open — a current 

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



1. Du Bois Key (fig. 84). — It consists of a 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. (i.) When the key is closed the current is made, and when it is 

 0}>ene'l the current is broken (fig. 85). Apparatus. — Daniell's cell 

 and detector, three wires, and a Du Eois key screwed to a table. 



(a.) As in the scheme (fig. 85) connect one wire from — pole of 

 the battery to one brass bar of the key. Connect the other brass 

 bar Avith one binding screw of the detector. Connect by means of 

 the third wire the other binding screw of the detector with the + 

 pole of the cell. 



(h.) On depressing the key (z.e., making the circuit) the needle 

 is deflected, on raising it {i.e., breaking the circuit) the needle 



