DOVE ON THE ELECTRICITY OF INDUCTION. 169 



posite direction, which consequently weakens the action of the 

 priraaiy current. At this moment the position becomes vertical, 

 the spring 3) comes in contact with the inserted piece of wood u 

 ui)on the cylinder Wa, the priniaiy current of the keeper a ceases, 

 and there is then excited an extra current E in the spiral S, 

 when the latter forms one continuous connected whole, which 

 is in the same direction with, and increases the action of the pri- 

 mary current. If the formation of this second extra current in 

 the same direction with the primary current is to be prevented, 

 then, the instant connexion is broken at u, the extra spiral S 

 must be removed from the closing connexion. This is effected 

 when I and III are united. If, on the contrary, I and II are 

 connected, we then obtain the primary current p weakened by 

 the influence of the incipient extra current A, which, circulating 

 in an opposite direction, is produced during the rotation of the 

 keeper from 0° to 90°, and augmented by the action of the final 

 current E, which, circulating in the same direction with the pri- 

 mary current, is excited when connexion is broken at u. In which 

 direction the final action is exerted, i. e. whether ^^ — A + E is 

 greater or less than x>, can be ascertained by inserting in place of 

 the spiral S a length of wire not forming a spiral, but offering 

 an equal amount of resistance to conduction*. The connexion of 

 I with II gives therefore the action of the primary current alone. 

 If, lastly, II and III are connected, we obtain, when S is an un- 

 coiled wire, no physiological action ; when S is a spiral, on the 

 contrary, a current in the same direction with the primary cur- 

 rent, i. e. the action of the final extra current by itself. 



71. With a straight wire inserted we obtain therefore for phy- 

 siological tests — 



With the connexion I and II the current j9. 

 I and III the currently. 

 II and III no current. 

 With an inserted extra spiral, on the contrary, — 



For the connexion I with II the current j^ — A + E. 

 I with III the current 7? —A. 

 II with III the current E. 



That no physiological action is obtained with the connexion II 



• The most convenient for this purpose is n thin German-silver wire, bent 

 up and down as the lines in the letter N, or ihu resistance-measurer recom- 

 mended by Wheatstone, consisting of a wooden and a metallic screw, upon 

 u'liith the wire coils and uncoils. 



N 2 



