168 DOVE ox THE ELECTRICITY OF INDUCTION. 



in an insulated manner upon the common axis of rotation ; the 

 cyUnders Wg and tv.^, on the contrary, are directly fixed upon it, and 

 are therefore in conducting connexion Avith it. The spiral S S, 

 which may be called the extra spiral, was composed, when it is not 

 otherwise expressly noticed, of two coils of well-varnished insu- 

 lated copper wire, each 400' in length, of which only one is re- 

 presented in fig. 7. The thickness of the wire is half a line, the 

 internal diameter of the coil from 2i" to 4f ". These two spirals 

 can be connected in a uniform manner or crossways. As it is well 

 known that this has no influence upon the extra current, this 

 arrangement affords us a simple mode of ascertaining whether 

 we have really to do with this cuiTent or not. Into the three sup- 

 ports I, II, III, wires are screwed, either two of which may be 

 connected by means of handles* through the body, or by the 

 voltameter or galvanometer, as has already been mentioned. 



The apparatus is therefore arranged in the manner represented 

 below, where a represents the rotating keeper g^ ]m: s 



with its coils, s the extra spiral, u the inter- 

 ruption by means of the intermittent spring 

 3) upon the cyHnder w^ and lastly, I, II, III 

 the wires leading to the apparatus for mea- 

 suring the current. These last admit of three 

 different modes of connexion, namely, I with 

 II, I with III, and II with III. In the first mode the keeper 

 and the extra spiral are in the circuit, in the second the keeper 

 only, and in the last only the spiral. 



70. After this detailed description of the apparatus, it will be 

 easy to account for that which occurs when the keeper revolves. 

 During the rotation of the keeper from 0° to 90°, i. e. from its 

 horizontal position before the poles of the magnet to the vertical 

 position at right angles to the line connecting the poles, the sur- 

 rounding wire of the keeper is throughout in metallic connexion, 

 for the spring 3) is always in contact with metal upon the cylinder 

 Wg. The increasing intensity of the primary current in the wire 

 p excites in the spiral S an extra current A circulating in an op- 



• The so-called gold strings intertwined with metal, which are used to fasten 

 the handles in the common Saxton's machine, must never be used when the in- 

 tensity of the physiological action is to be determined ; for the intensity of the 

 shock depends essentially with these upon the amount of force with which the 

 strings are stretched. Spirally coiled cop])er wires firmly clamped with screws 

 which are sufficiently elastic and always etTect a uniform contact, are to be pre- 

 ferred. 



