2l8 THE SCIENTIFIC PAPERS OF 



increase that resistance simply by reducing the diameter of the 

 wire, and increasing the number of convolutions until the same 

 thickness of coil was obtained, the magnetic excitement and with 

 it the electro-motive force of the current produced at a given 

 velocity of rotation would suffer a material decrease. The current 

 flowing through the helix coil would moreover have tp divide 

 itself, and in order to reach the same limit in the outer circuit its 

 intensity in the helix coil would have to be increased, causing- 

 it to heat more readily than before. It was necessary, therefore, 

 to raise the effect of the magnet current to the same level as 

 before with as small a proportion of the helix current as possible, 

 in order to leave a maximum proportion of the current for the 

 outer circuit. In order to effect this, the magnet bars had to be 

 increased in length, and placed further apart so as to provide room 

 for coils of greatly increased weight and dimensions ; at the same 

 time the helix wire had to be increased in diameter to give room 

 for the aggregate current, but in reality I found it advantageous 

 to increase the diameter of the same in a much greater pro- 

 portion. 



These general conditions having been determined by preliminary 

 experiment, Mr. Lauckert, electrician engaged at my works, under- 

 took a series of comparative experiments which are given in 

 the appendix * attached to this paper, and the results are given 

 numerically and exhibited in curves.* On examining the curves it 

 will be remarked : 



1. That the electro-motive force, instead of diminishing with 

 increased resistance, increases at first rapidly, then more slowly 

 towards an asymptote. 



2. That the current in the outer circuit is actually greater for a 

 unit and a-half resistance than for one unit. 



3. With an external resistance of one unit, which is about 

 equivalent to an electric arc when 30 or 40 webers are passing 

 through it, 2'44 horse-power is expended, of which 1 -21) horse- 

 power is usefully employed : an efficiency of 53 per cent, as- 

 compared with 45 per cent, in the case of the ordinary dynamo 

 machine. 



4. That the maximum energy which can be demanded from the 



* It has not been considered necessary to reproduce these. 



