mth a Description of a Magnetic Contact-breaker. 19 



induction, by wliich we are enabled to render the electricity 

 naturally present, but previously latent, in a coil of wire, ob- 

 vious, by allowing a current from a small voltaic pair to circu- 

 late through a second coil placed in a direction parallel to the 

 first. Perhaps the best mode for effecting this is the follow- 

 ing, which my own experiments have induced me to adopt: 

 Wind on a reel with a hollow axis, three inches in length, 

 about 60 feet of copper wire -^^ inch in diameter, covered 

 with cotton thread for the purpose of insulation. Allow the 

 two ends of the wire to project from the reel, and call them 

 A and B. Over this coil, wind a second insulated copper 

 wire -^-Q inch in diameter and about 1500 feet in length; let 

 the two ends of this second coil be called C and D. It is 

 now evident from the law of electro-dynamic induction that 

 on connecting the ends A and B of the thick coil with a single 

 pair of plates, a current of electricity is set in motion in the 

 thin coil ; and on breaking contact, a second current in 

 another direction traverses the same coil, sufficient in intensity 

 to communicate an energetic shock to any person grasping 

 the ends C and D of the long helix ; and by very rapidly break- 

 ing contact a rapid series of intense shocks may be communi- 

 cated to any conducting body connecting the ends C and D. 



As it is exceedingly inconvenient to effect the rupture of 

 contact with the battery by raising the end of the thick coil 

 with the hands, various modes have been proposed, depend- 

 ing all however upon the same principle, viz. that of a spring 

 pressing against a revolving cog wheel, or some modification 

 of this arrangement. These instruments all present the same 

 objection, that of the hands of the operator being employed in 

 turning a wheel for the purpose of breaking contact when they 

 are required elsewhere to direct the induced current through 

 any conducting body he pleases; this difficulty may, it is true, 

 be obviated by causing an assistant to turn the wheel, but even 

 this is highly inconvenient when currents are required for 

 electrolytic purposes during half an hour, or even a less space 

 of time. 



For these reasons, together with many others depending 

 upon the irregular rotation of a cog-wheel, I have disused 

 instruments depending upon this principle for the last ten 

 months, and have adopted one which appears to me to be 

 quite free from the above objections, and is certainly an ex- 

 ceedingly simple and convenient one, by the aid of which a 

 series of induced currents may be passed through any con- 

 ducting body for almost any length of time. 



This instrument consists of a base-board about eight inches 

 in length and three in breadth, furnished at both ends with a 



D2 



