Instruments and Experitnents. 107 



ished by inserting them in a tube of glass or other non-conduct- 

 ing substance, before introducing them into the hehx. 



When a bar of iron is contained in the hehx, and a small key 

 or some nails are applied to one end of it, notwithstanding its 

 magnetic attraction is intermitted every time the voltaic circuit 

 is broken, yet, it being almost instantaneously renewed, they do 

 not cease to be sustained. This experiment succeeds best when 

 the iron bar is enclosed in a brass tube previously to being intro- 

 duced into the helix, the closed circuits of the tube tending to 

 prolong its magnetism. 



The double helix and electrotome, in consequence of being 

 provided with a mechanical contrivance for breaking the battery 

 circuit, may be used with a very small battery, although its effects 

 are of course most striking, when used with a powerful one. If 

 a voltaic pair, consisting of a silver dollar and a piece of rolled 

 zinc of the same size be used, and the helix be filled with soft 

 iron wires, the shock is quite severe. 



Water may be decomposed by connecting the outer helix with 

 an instrument for that purpose having very small platinum wires 

 guarded with glass, as originally used by Wollaston. The ex- 

 tremities of the platinum wires, while the decomposition is going 

 on, appear in a dark room, one constantly and brightly, and the 

 other intermittingly and feebly luminous. If the apparatus for 

 decomposition is removed out of the noise of the double helix 

 and electrotome, rapid discharges are heard in the water, produ- 

 cing sharp ticking sounds, audible at the distance of eighty or 

 a hundred feet, and synchronous with the ruptures of the vol- 

 taic circuit. Decomposition is effected both by the initial and 

 terminal secondary currents, that is to say, by the currents indu- 

 ced both on completing and on breaking the battery circuit ; but 

 the ticking noise and sparks accompanying the rapid discharges 

 in the water, are produced only by the terminal secondary cur- 

 rent. Hydrogen may be kindled and brilliant scintillations pro- 

 duced by the double helix and electrotome. A Leyden jar, the 

 knob of which is connected with the inside coating by a contin- 

 uous wire, may be feebly charged, and slight shocks be rapidly 

 received from it, by bringing the knob in contact with one of the 

 cups of the outer helix, and grasping with the two hands respec- 

 tively the outer coating of the jar and a handle connected with 

 the other cup. The instrument is likewise very convenient for 



