1 1 6 ELECTRIC LIGHTlN (/. 



The conclusions of Elihu Thompson and J. Hudson are 

 however, somewhat different, and in our opinion more 

 correct. They are separately stated in this way : 



i. The Gramme machine is the most economical as a 

 means of converting motive force into electric currents ; it 

 utilizes in the arc from 38 to 41 per cent, of the motive work 

 produced, after deduction is made for friction and the resistance 

 of the air. In this machine the loss of power due to friction 

 and to local actions is smallest, doubtless on account of its 

 smaller speed. If the arc is maintained in its normal condition 

 scarcely any heating is produced in the machine, and the pre- 

 sence of sparks at the commutator can hardly be perceived. 



2. The large Brush machine comes second in the order of 

 efficiency. It produces in the luminous arc useful work equiva- 

 lent to 31 per cent, of the motive power employed, or to 37^ per 

 cent, after the friction has been deducted. It is, in fact, but 

 little inferior to the Gramme machine, but it has the disadvan- 

 tage of requiring a very high speed, and consequently there 

 occurs a greater loss on account of friction. This disadvantage 

 is, it is true, compensated by its power of working with a greater 

 external resistance compared with the internal resistance of the 

 generator, and this, in a manner, does away with the heating of 

 the machine. It is this machine, moreover, which has given 

 the most powerful light and the most intense currents. 



3. The small Brush machine ranks third, with a useful work 

 in the arc estimated at 27 per cent, of the motive power em- 

 ployed, or 31 per cent, with deduction for friction. Although 

 this machine is in some respects inferior to the Gramme ma- 

 chine, it is nevertheless admirably contrived for the production 

 of currents of intensity, and it can be made to produce currents 

 of very different electro-motive force. By suitably arranging 

 this machine, the electro- motive force maybe increased from 55 

 to 120 volts, and the currents it produces may be divided between 

 two circuits, an advantage, however, which is possessed by 

 other machines. The simplicity of the commutator and the 

 facilities for repairs which the arrangement allows, are also 

 advantages to be taken into consideration. This machine, 

 moreover, does not produce much heating. 



4. Wallace Farmer's does not yield in the working circuit 



