OF ARTS AND SCIENCES. 129 



tilled mercury was used in all cases : it answered the double purpose 

 of conducting the current and bringing the vibrations quickly to rest. 



Through the courtesy of Captain Breese, U. S. N., in charge of the 

 U. S. Torpedo Station at Newport, R. L, who obtained permission 

 for me to use the dynamo-electric engines at that place, I was enabled 

 to make a series of measurements with the dynamometer described 

 above. 



The resistances used consisted of large bands of german silver, each 

 in the neighborhood of t X q of an ohm resistance. The foot-pounds 

 of work consumed were measured by a Batchelder's dynamometer,* 

 which is fully described in Dingler's " Polytechnic Journal," 1844, 

 Vol. II. This dynamometer is not suitable for the measurement of 

 small or great horse-power ; but it answered very well in the limits of 

 velocities and horse-powers to which I confined myself. An accurate 

 measure of the work consumed in running a dynamo-electric machine 

 upon a closed circuit would require the use of gearing instead of belt- 

 ing ; for it is difficult to estimate the slip of the belting. On account 

 of the error introduced from this latter-mentioned clause, I have given 

 the whole work required to run each machine on a closed circuit. 

 The slip on an open circuit would be small, but on a closed circuit 

 might be very large. The machines were run under the same condi- 

 tions of shafting and pulleys. It was estimated that the Siemens 

 required 0.031 horse-power on an open circuit, and the Gramme 

 0.206 to 0.328 horse-power. The term efficiency denotes the ratio of 

 the equivalent in metre grammes of the current produced to the metre 

 grammes consumed in running the dynamo-electric machine. Since 

 one veber through one ohm 



10 5 



the work w = C 2 Et = (10 ~ 2 ) 2 X 10 7 = 10 3 = 1000 units of work, 

 and dividing by the unit employed we have 



equivalent of 1 Veber =102 metre grammes, 

 one foot-pound =138 metre grammes. 



* Eor which I am indebted to the Massachusetts Institute of Technology. 

 vol. xiv. (n. s. vi.) 9 



