10 



EXPERIMENTAL PHYSIOL' )Q Y 



distance between the two coils. The value of the induced current 

 may be determined empirically by use of the galvanometer. Some 

 forms of coil are already graduated in this manner. 



Another plan which is at times adopted for varying the strength 

 of the induced current is to have the secondary coil so fitted that it 

 can be rotated and its long axis set at any angle to the axis of the 

 primary. The induced current, with a fixed alteration in the primary, 

 is then proportional to the cosine of the angle between the two axes 

 of the coils. 



SOME FORMS OF KEYS FOR OPENING AND 

 CLOSING A CIRCUIT 



The MERCURY KEY.— This key is used for making and breaking 

 a current by hand, and is constructed in various forms (see figs. 11 

 and 12). 



In fig. 11 there are two cups, c 1 , c 2 , hollowed out in a vulcanite 

 base and with two binding screws, b 1 and b 2 , entering them from the 



B Inst. Co. Ltd. <Af>M3. 



Figs. 11 and 12. — Two Forms of Mercury Key. 



side. The cups are nearly filled with mercury, and can be connected 

 by means of the stout bent copper wire w w which hinges through a 



piece of vulcanite e. In fig. 12 

 there is a single mercury cup into 

 which a wire dips to make contact 

 with the binding screw. 



The SPRING or CONTACT 

 KEY (fig. 13) consists of a metal 

 spring connected to a binding 

 screw a. At its movable end there 

 is a vulcanite knob c by which it can be depressed, and thus a platinum 

 point on its lower surface brought into contact with a platinum plate 



Fig. 13. —Simple Form of Spring Key. 



