71 



34« Prof. Dove's Experiments in Magneto-Electricity. 



however, be entirely overcome, so that perfectly corresponding 

 experiments may be made with the extra-current both at its 

 commencement and cessation. 



In the following figure, let a represent the rotating anchor 

 of a Saxton's machine, s a spiral con- 

 nected with the wire of this anchor, for 

 the production of the extra-current, and 

 so arranged as to allow of iron being 

 inserted into it, u the place where the 

 wire opens by means of the spring when 

 the anchor is in a perpendicular posi- 

 tion. I, II, III, three conductors which 

 can be close by any body, as a galvanometer, voltameter, &c. 

 This arrangement allows of three kinds of junction, viz. I 

 and II, I and III, II and III. 



If we call the primary current p, and designate by A 

 the reversed extra-current formed at its commencement, and 

 by E the similar (in direction) extra- current produced at its 

 cessation, we find as follows : — As long as the wire at u re- 

 mains closed, the intensity of p increases during the rotation 

 of the anchor from 0° to 90°, that is, p produces the current 

 A, and we obtain^? — A. If we close I and III by means of 

 the body (which extra connexion does not experience the 

 least physiological effects as long as u is closed), it receives 

 the shock of the current p— A, inasmuch as E cannot be 

 formed, because at the moment u is opened it passes out 

 of the connexion ; if we close II and III we obtain E ; if a 

 straight wire is substituted for s there is no action : by closing 

 I and II we get p— A + E, as when u opens the extra-current 

 is formed in s. The presence of iron in the spiral s produces 

 the following effects : — 



I and III) {p — A}. The shock with an empty spiral is 

 much greater than when none is inserted, i. e. p—A smaller 

 than p ; when iron is inserted it is much weaker, for A is in- 

 creased while p remains unchanged. 



II and III) {E}. The shock is strengthened by the inser- 

 tion of iron. 



I and II) {p— A + E}. The shock is much stronger than 

 with I and III, for p — A + E is greater than p— A ; it remains 

 almost unchanged when iron is inserted, because E increases 

 almost the same as A. 



The opening spark at u is considerably weaker when iron 

 is put into the spiral, but recovers its intensity if II and III 

 are metallically connected. The spark between II and III 

 or I and III is increased in intensity by the presence of iron ; 



