344 Lord Rayleigh on the Measurement of Alternate Currents 



length of the needle. The deflecting couple is thus evanescent 

 when the needle stands either parallel or perpendicular to the 

 magnetizing force, and rises to a maximum at the angle of 

 45°. For practical purposes the law of proportionality to the 

 mean square of current would seem to be trustworthy so long 

 as no great change occurs in the frequency or type of current; 

 otherwise eddy currents in the iron might lead to error, unless 

 the metal were finely subdivided. 



It is hardly to be supposed that for ordinary purposes a 

 suspended iron needle would compete in convenience with the 

 excellent instruments now generally available ; but having 

 found it suitable for a special purpose of my own, I think 

 it may be worth while to draw to it the attention of those 

 interested. In experiments upon the oxidation of nitrogen by 

 the electric arc or flame it was desired to ascertain the relation 

 between the electric power absorbed and the amount of 

 nitrogen oxidized. A transformer with an unclosed magnetic 

 circuit was employed to raise the potential from that of the 

 supply to the 3000 volts or more needed at the platinum ter- 

 minals. Commercial ampere-meters and volt-meters gave with 

 all needed precision the current and potential at the primary 

 of the transformer; but, as is well known, these data do not 

 suffice for an estimate of power. The latter depends also 

 upon the angle of lag, or retardation of current relatively to 

 potential-difference. If this angle be 0, the power actually 

 employed is to be found by multiplying the product of volts 

 and amperes by cos 6, so that the actual power may be less 

 to any extent than the apparent power represented by the 

 simple product. Various watt-meters have been introduced 

 for measuring the actual power directly, but I could not hear 

 of one suitable for the large current of 40 amperes used at 

 the Royal Institution. Working subsequently in the country 

 I returned to the problem, and succeeded in determining the 

 angle of lag very easily by means of the principle now to be 

 explained. 



The soft iron needle of 2 centim. in length, suspended by 

 a fine torsion-fibre of glass and carrying a mirror in the usual 

 way, is inclined at 45° to the direction of the magnetic force. 

 This force is due to currents in two coils, the common axis of 

 the coils being horizontal and passing through the centre of 

 the needle. As in ordinary galvanometers, the mean plane of 

 each coil may include the centre of the needle ; but it was 

 found better to dispose the coils on opposite sides and at dis- 

 tances from the needle which could be varied. A plan of the 

 arrangement is sketched diagramatically in the woodcut, where 

 MM, BS represent the two coils, the common axis HK passing 



