170 Prof. J. Trowbridge on Methods of Measuring 



dipped in the mercury were tipped with aluminium; but 

 when a strong current passed through them the mercury was 

 disturbed by an apparent ebullition, and the mercury was 

 speedily covered with a black deposit. It was found that 

 copper points would answer perfectly well. Distilled mercury 

 was used in all cases ; it answered the double purpose of con- 

 ducting 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. I., 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 neighbourhood of j 1 ^- of an ohm resistance. 

 The foot-pounds of work consumed were measured by a Bat- 

 chelder'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 gear- 

 ing instead of belting; for it is difficult to estimate the slip of 

 the belting. On account of the error introduced from this latter- 

 mentioned cause, 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 conditions 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-elec- 

 tric machine. Since one veber through one ohm 



10 5 

 — O — ^L — io-2 



the work *r=C 2 R* = (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, 



1 foot-pound = 138 „ 



* For which I am indebted to the Massachusetts Institute of Techno- 

 logy. 



