If these conditions are not fulfilled, the wattmeter readings, assuming the wattmeter to have been calibrated with continuous currents, may be either too high or too low when alternating currents are being used.
The calorimeter tube was calibrated by a thread of mercury weighing 19 milligrams, which occupied eighty-five divisions.
Hence if the galvanometer is calibrated by a potentiometer we can determine the value of this current in amperes, and knowing the value of n and V thus determine C. Various forms of commutator have been devised for effecting this charge and discharge rapidly by J.
The wattmeter can therefore be calibrated so as to give direct readings of the power reckoned in watts, taken up in the circuit; hence its name, wattmeter.
For such purposes the whole of the working parts are contained in a metal case; the indicating needle moving over a divided scale which is calibrated to show directly the potential difference in volts of the terminals of the instrument.