To measure an alternating-current resistance and compare it with the direct-current resistance — Electrodynamometer method

For comparing an alternating-current and a direct-current resistance by the electrodynamometer method in a precise manner the apparatus required is a frequency meter to measure the frequency of the current used (which must be known, as the quantity being measured will vary with frequency), an alternating-current ammeter to give roughly the value of the current (for the alternating-current resistance will also, in general, depend upon the value of the current), a three-point double-throw switch for quickly changing connections, resistances, and an electrodynamometer. This last piece of apparatus should have sufficient capacity in its current coils to carry, without heating, the full current. Its hanging or potential coils should be two in number, and so arranged as to form a system which is perfectly astatic in respect to the earth's field. The constant of the instrument will then be the same for direct and alternating currents. All good electrodynamometers are constructed in this way. Either the Rowland deflection type or Siemens type, constructed to be astatic, may be used. The method to be described was tested with a Rowland deflection type electrodynamometer.