276 Magneto-Electric Multipliei\ 



or diameter ; for if longer, the armature will be unnecessarily 

 long and retain a considerable degree of magnetism, which would 

 be equivalent in the result to the retention of magnetism by the 

 magnet itself; and if the diameter be increased, it will be dispro- 

 portionate to the length. The magnet &c, is compound, and made 

 of the best annealed soft iron wire. No. 18. It is covered with 

 four superficial helices of No. 16 copper wire, and exterior to this, 

 about four hundred feet of fine copper wire. The armature a is 

 slightly curved, and its extremities are made to pass very near 

 the poles of the magnet. At a, upon the revolving shaft, is placed 

 the electrotome, made of copper or silver, into which are fitted 

 segments of wood or ivory to intercept the galvanic current at 

 proper intervals ; p, 7i, are the cups to receive the wires comple- 

 ting battery connexion. The wire, n, rests with a slight spring 

 upon the electrotome ; p, is one termination of the large wires of 

 the magnet. The other magnet wire, c, springs against the shaft 

 at d, so as to be in metallic communication with o and n ; r, s, 

 are the fine wire terminations from which the shocks are received ; 

 they may at pleasure be inserted in mercury cups screwed to the 

 top of the pillar. The cups are convenient in certain experi- 

 ments, but if the wires are left as in the figure, when the machine 

 is in action, a beautiful purple light plays between their quiver- 

 ing extremities, provided they previously touch ; m, is the mul- 

 tiplying wheel, with its band running upon a small pulley on the 

 shaft. The electrotome, o, is set upon the shaft so that its me- 

 tallic portions shall separate from the wire spring, n, immediately 

 after the extremities of the armature, a, have passed the poles of 

 the magnet. The electrotome may also be so adjusted, that the 

 armature will revolve of itself when a large battery is applied. 

 The most interesting and important application of this ingtriunent 

 is to obtain a spark and shock from a single thermo-electric pair. 

 A common pair of bismuth and antimony plates, heated and 

 cooled, are connected with the cups 7i, p, and as the wheel is 

 turned, a shock is felt as far as the wrist and sometimes to the 

 elbow, by touching r, s, with wetted metallic handles. The 

 spark is seen at o, upon the electrotome. When the shock is de- 

 sired, the electrotome is oiled ; when the spark is to be observed, 

 the electrotome is to be cleaned, and if of copper, rubbed with 

 some fine amalgam. The sparks and shocks occur at each sepa- 

 ration of the armature and magnet. 



Washington, July 23d, 1839. 



