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of wires of the same length around the same bar, so as to lessen the 

 resistance which the galvanic current experienced in passing from the 

 zinc to the copper through the coil. To bring this to the test of experi- 

 ment, a second wire, equal in length to the first, was wound around the 

 last mentioned magnet, and its ends soldered to the plates of the same 

 battery. 



The magnet with this additional wire lifted twenty eight pounds, or, in 

 other words, its power was doubled. 



A series of experiments was afterwards made, to determine the resist- 

 ance to conduction of wires of dijGFerent lengths and diameters, and the 

 proper lengths and number of wires for producing, with different kinds of 

 galvanic batteries, the maximum of amount of magnetic development 

 with a given quantity of zinc surface. For this purpose a bar of soft 

 iron, two inches square and twenty inches long, weighing twenty-one 

 pounds, and much larger than any before used, was bent in the form of 

 a horse-shoe. Around this were wound nine strands of copper wire, each 

 sixty feet long, the ends left projecting so that one or more coils could 

 be used at once, either connected with a battery or with each other, thus 

 forming several coils with several battery connections, or one long coil 

 with single battery connections. The greatest effect obtained with this 

 magnet, using a battery of a single pair, with a zinc plate of two-fifths of 

 a square foot of surface, and all the wire arranged as separate coils, was 

 to lift a weight of six hundred and fifty pounds ; with a large battery the 

 effect was increased to seven hundred and fifty pounds. In a subsequent 

 series of experiments, not published with the preceding, the same magnet 

 was made to sustain one thousand pounds. When a compound battery 

 was employed of a number of pairs, it was found that the greatest effect 

 was produced when all the wires were arranged as a single long coil. I 

 subsequently constructed electro-magnets on the same plan, which sup- 

 ported much greater weights. One of these, now in the cabinet of 

 Princeton, will sustain three thousand six hundred pounds with a battery 

 occupying about a cubic foot of space. It consists of thirty strands of 

 wire, each about forty feet in length. 



The abovementioned experiments exhibit the important fact that when 

 a galvanic battery of intensity (that is to say, a battery consisting of a 

 number of pairs) is employed, the electro-magnet connected with it must 

 be wound with one long wire, in order to produce the greatest effect ; and 

 that when a battery of quantity, (that is, one of a single pair,) is employed, 

 the proper form of the magnet connected with it is that in which several 

 shorter wires are wound around the iron. The first of these magnets, 

 which is the one now employed in the long or main circuit of the tele- 



