Electro-Measuring Instruments. 305 



employment of a single cell of known electromotive force but 

 of totally unknown resistance. 



Objections to the Employment of a Constant. — But these 

 instruments possessed one serious fault looked at from the 

 user's point of view, and two others which especially concerned 

 the manufacturer. The scales of all the instruments were 

 divided into degrees; and although the deflections were pro- 

 portional respectively to the current or the potential difference 

 to be measured, still there was nothing to show to how 

 many amperes or volts one degree of the scale corresponded. 

 Consequently for each instrument there w r as a " Calibration 

 Constant," such as 1*75 ampere or 2*14 volts per degree, 

 which was determined experimentally by the makers. We 

 have here two of such old type of voltmeter, in one of which 

 5*7 volts produce one degree deflection, and in the other 

 0*09 volt produces one degree • the former being intended 

 for measuring electromotive forces up to about 250 volts, the 

 other the electromotive force of a single cell. But the em- 

 ployment of such a constant had the very serious objection 

 that, as all the instruments looked exactly alike, there was the 

 great danger in any factory, where several of these instru- 

 ments were in use, of the constant for one instrument being 

 used accidentally for that of another. And this mistake not 

 only renders the answer, of course, quite wrong, but is liable 

 to lead to the destruction of the instrument, or at any rate to 

 the breaking of the pointer, from, say, an instrument having a 

 constant 0*12 ampere per degree being put on a circuit 

 through which 50 amperes are flowing, in consequence of the 

 experimenter mistaking the "constant" for 1*2 ampere per 

 degree. Further, even if no mistake be made in the " con- 

 stant," it is troublesome, except for those who are expert in 

 mental calculation (a quality not always predominating in the 

 men in the charge of electric-light circuits), to multiply a 

 " constant " 3*17 by the deflection, say 21*32, quickly without 

 the employment of paper and pencil. 



We therefore decided, not merely to avoid using tables of 

 values such as had to be employed by M. Deprez, but even 

 to abandon the use of constants altogether, and to arrange 

 matters so that the pointer pointed at once throughout the 

 whole range of the scale to the number of amperes or volts to be 

 measured ; to design, in fact, " Direct- Reading " instruments. 

 And for the future we propose to confine the definition of an 

 "Ammeter," a " Voltmeter," and an " Ohmmeter " to instru- 

 ments on which respectively amperes, volts, and ohms can be 

 immediately read off without any calculation or any reference 

 to a constant or table of values. 



