04 TRANSACTIONS OF THE [DEC. 21, 



stitute. Its extreme capacity is 100 horse-power, and yet, while 

 making 1,040 revolutions per minute, carrying a load of 29 

 horse-power, it was possible to measure with certainty the differ- 

 ence of power required by an Edison volt meter requiring j\ of 

 a horse-power. It announced at once the making and breaking 

 of the circuit of this volt meter, measuring the work lost in it 

 with accuracy. Still other tests showed its capability to promptly 

 register small changes of power while carrying great loads, and 

 proved that the slight and rapid jar of the parts, clue to a high 

 speed, increased its sensitiveness of measurement. 



Finally, this dynamometer was calibrated by the agitation 

 of water, heating something over 5 tons of water through 15.5° 

 centigrade, giving, as the mechanical equivalent of heat, 772,81 

 foot lbs. per British unit of heat (see report). 



"While little can be claimed in the way of originality of appa- 

 ratus or methods used in the electrical measurements of these 

 tests, I trust tluxt an examination of tlie precautions will convince 

 you of the extreme care taken to obtain correct results, (See 

 ''Competitive Tests of Dynamo Electric Machines," Journal 

 Franklin Institute, Nov., 1885.) The dynamo electric machine 

 has grown out of the fact that, if we move a dead wire in the 

 field of another fixed wire, through which a current is passing, 

 the dead wire will have a current generated in it whose electro- 

 motive force is proi)ortional to, first, the intensity of the current 

 in the fixed wire; second, to the velocity of motion of the mov- 

 ing wire, and third, to the acting length of the moving wire. 



If I take a single wire. Fig, 2, and |>ass a current through it, 

 its field will resemble a whirl[)0()l of whicli the wire is the centre. 



If I take two wires and place them a short distance apart, Fig. 

 3, and pass a current in tlie same direction through both, their 

 fields will combine to form an oval field, and any number of ad- 

 jacent wires, with currents in the same direction, will do the 

 same thing, forming a field of an intensity proportional to the 

 number of wires and the intensity of the current in each. The 

 field would, however, be of loose texture, so to say, and the lines 

 of force far apart if the lines of force formed themselves around 

 the wires in the air. 



Iron, because of its great permeability to the lines of magnetic- 



