A new potentiometer for the measurement of electromotive force and current
Herbert B. Brooks · Bulletin of the Bureau of Standards · 1906
Instruments for the measurement of electromotive force and cur- rent may be divided into two main classes.In those of the first class, which may be called deflection instruments, a movement of one element of the instrument is produced against an opposing force, an indication of the quantity to be measured being given by the extent of the motion, as shown by an index moving over a graduated scale.To this class belong many types of ammeters and voltmeters, such as the moving coil, the magnetic vane, electro- dynamometer, hot wire, and electrostatic instruments.Those of the second class may be called balance instruments.In these the electric quantity to be measured exerts a force which is opposed by another force, an adjustment of the instrument being made until no motion results, when the quantity to be measured is determined by the extent of the necessary adjustment.To this class belong Wheatstone bridges, potentiometers, and Kelvin balances.Each of these two classes of measuring instruments has its advan- tages and disadvantages.The deflection instrument can not be read, as a rule, with an accuracy greater than one part in a thousand; while on account of zero errors, inaccuracy of the scale, heating due to the current, changes of external temperature, stray magnetic fields, and other disturbing influences, the accuracy really obtained in the result is much lower than this.It is possible, of course, to determine and allow for some of these errors, but others, such as those due to stray field, are not easily allowed for.The balance instrument can usually be read with greater accuracy, and in the 225