on various Phcenomena of Induction. 277 



considered correct. Care must be taken to prevent the ad- 

 ditional wires from being heated by the current; without this 

 their conductibility with small lengths ceases to be comparable 

 to their conductibility with great lengths. The current of in- 

 duction which traverses the rheometer should, as much as pos- 

 sible, be moderated, so that it shall not alter the relation of 

 the magnetization of the two needles. The rheometer itself 

 should be removed from the inductric spiral, to a sufficient di- 

 stance for the electro-maffnetic induction of its inductor wires 

 not to be felt. 



ZB. In addition, the two extremities of the wires, the im- 

 mersion of which must close the simple or double inductor 

 circuit, is to be plunged at the same instant into the mercury. 

 It is convenient to destroy the contact at the precise instant 

 when the needle tends, by virtue of the acquired movement, 

 to go beyond its primitive position of equilibrium ; we thus 

 avail ourselves of the second induced current to render it im- 

 moveable, which saves much time in long experiments of this 

 nature, and destroys the influence of the variations of intensity 

 of the current in piles continued in action during two or three 

 hours. 



§ IV. Influence of the state of closing or opening of the circuit 

 indtwed upon the action of the inductor current on itself. 



36. There is a very great correlation between the inductor 

 circuit and the induced circuit. When the latter is open, we 

 know that we obtain, at the breaking of the inductor current 

 in the mercury, very brilliant sparks, the brightness of which 

 increases if we place a hollow pencil of iron wires in the helix, 

 and still more if this pencil is solid. These sparks, impro- 

 perly so called, are, I think, produced by the combustion and 

 the volatilization of the globule of mercury which adheres to 

 the extremity of the wire, and which becomes thinner in pro- 

 portion as this extremity is the more removed from the sur- 

 face of the liquid in its reservoir. Now, when the induced 

 circuit is very accurately closed, these sparks diminish consi- 

 derably, and even disappear altogether*. Between the two 

 limits of open circuit and circuit perfectly closed by the aid 

 of very short massive and good conducting bodies, there is an 

 infinity of intermediate degrees to which sparks more or less 

 brilliant correspond. 



* M. Abria has also shown, that a circuit placed near the inductor does 

 not exert any reaction when it is open. See Jnn. dc Ch. et de Phys., vol. iii. 

 p. 10 (September 1841). [We believe that it is perfectly well known to 

 British obsei-vers that the sparks here alluded to are produced solely by the 

 combustion of the mercury. — Edit.] 



