102 DYNAMO-ELECTRIC MACHINES. 



The means employed for knowing the capacity of a machine, 

 as well as for the measure of work it can accomplish, and, also, the 

 measure of the work it has to overcome, is an instrument, usually 

 small enough to be carried in the pocket, but sometimes of very 

 large dimensions. They are made to measure very feeble currents 

 of such small electro-motive as the one millionth part of a volt, or 

 higher than the currents generated by a Brush machine. Such 

 instruments, as you are probably aware, are called galvanometers. 

 They are sometimes called, when used as a measure of quantity of 

 the electric generators under discussion, current meters. Some of 

 them are very complex, and require a knowledge of the higher 

 mathematics to understand the principles upon which they are based, 

 while others are so simple as to require no more than a knowledge 

 of simple ratios. Of the latter class is that known as Deprez's gal- 

 vanometer, a very crude sample of which has been brought here. 

 This instrument is based upon the ratio of two opposing magnetic 

 forces, one of which is obtained from a permanent magnet, which is 

 a part of the instrument, and the other is created by the current to 

 to be measured. In form, it consists ot a horse-shoe magnet, made 

 up of five layers of flat steel, bent in the form ot the letter U, and 

 fastened to a board which serves as a base. In the strongest part 

 of the field, between the poles, is fastened a rectangular box, which 

 just fills the space between the limbs of the magnet, around which 

 is wound, for the measurement of currents of intensity, a very few 

 turns of coarse wire, or a single band of sheet copper. In the inside 

 of the box, upon two V shaped pieces of brass, one of which is fast- 

 tened to each end, rests a small thin bar of iron with its two ends V 

 shaped, cut along each edge like the teeth of a very coarse comb 

 to deduce friction, and wide enough just to clear the inside of the box. 

 At the end of this bar next to the binding posts, is inserted an 

 upright piece of brass wire, whose movements, to right or left, are 

 indicated and measured upon a quadrant, placed just behind it, and 

 arching the ends of the magnet. Two binding posts, with screws, 

 standing in front of the quadrant, to which are attached the ends 

 of the wire encircling the box, complete the machine. 



This instrument has been given the name of absolute galvan- 

 ometer, from its power of measuring the intensity and electro-motive 

 force of a current. Its action is as follows : — The permanent magnet- 



