in Magnetism and Electricity. 99 



390 lbs. The armature of this machine was coiled with an in- 

 sulated copper strand 84 feet in length, the weight of which was 

 about 28 lbs. 



64. When the electromagnet of the 5-inch machine was excited 

 by the 2|-inch magneto-electric machine, the armature of the 

 5 -inch machine being driven at about 2000 revolutions per 

 minute, the electricity from it melted 15 inches of No. 15 iron 

 wire 0*075 of an inch in diameter. Now it was found that a 

 piece of No. 15 iron wire 15 inches in length was about seven 

 times the weight of 8 inches of the wire melted by the 2J-inch 

 electromagnetic machine (61); and as the 5~inch machine was 

 about eight times the weight of the 2|-inch electromagnetic 

 machine, the increase of power of the 5 -inch machine, as mea- 

 sured by its heating-effects, appears to me to be nearly in direct 

 proportion to the increase of its cubical dimensions, after allow- 

 ance has been made for the diminished speed at which the ar- 

 mature was driven, together with the small increase of power 

 which might have been obtained had the electromagnet been 

 excited by a more powerful magneto-electric machine. 



65. For the purpose of learning to what extent the power of 

 the electromagnetic machine might be increased by an exaltation 

 of the magnetism of the electromagnet without changing the 

 speed at which the armature was driven, the electromagnet of 

 the 5 -inch machine was excited by the direct current from the 

 2J-inch electromagnetic machine (59), the latter being in its 

 turn excited by the 2^-inch magneto-electric machine (58). On 

 making, the experiment, it was found that although the mag- 

 netism of the electromagnet (63) was considerably increased, yet 

 this increase was only accompanied by a very small additional 

 quantity of electricity from the armature — thus showing that 

 the full power of the 5-inch machine had been very nearly 

 attained when its electromagnet was excited by the 2J-inch 

 magneto-electric machine alone. 



G6. Having found that an increase in the dimensions of the 

 electromagnetic machine was accompanied by a proportionate 

 and satisfactory increase of the magnetic and electric forces, a 

 10-inch electromagnetic machine was constructed ; and as its 

 calorific and illuminating powers are very remarkable, some par- 

 ticulars respecting the machine, together with a few experiments 

 made with it, may perhaps be found to possess some interest, 

 especially for those physicists who are engaged in determining 

 the quantitative relations existing between the various forces as 

 manifested to the senses. 



67. In describing the different parts of the machine, reference 

 will still be made to figures 1-9, which have been drawn to a 

 proportionate scale. A perspective view of the machine com- 



H2 



