H. A. Rowland — Electrical Measurement. 429 



Art. XLYII. — Electrical Measurement by Alternating Cur- 

 rents ; by Henry A. Rowland. 



The electrical quantities pertaining to an electric current 

 which it is usually necessary to measure, outside of current, 

 electromotive force, watts, etc., are resistances, self and mutual 

 inductances and capacities. I propose to treat of the measure- 

 ment of alternating currents, electromotive force and watts in 

 a separate paper. Resistances are ordinarily best dealt with by 

 continuous currents, except liquid resistances. I propose to 

 treat in this paper, however, mainly of inductances, self and 

 mutual, and of capacities together with their ratios and values 

 in absolute measure as obtained by alternating currents. I 

 also give a few methods of resistance measurement more accu- 

 rate than usually given by means of telephones or electrody- 

 namometers as usually used and specially suitable for resistances 

 of electrolytic liquids. 



I have introduced many new and some old methods, depend- 

 ing upon making the whole current through a given branch 

 circuit equal to zero. These always require two adjustments 

 and they must often be made simultaneously. However, some 

 of them admit of the adjustments being made independently 

 of each other, and these, of course, are the most convenient. 

 But all these zero methods do not admit of any great accuracy 

 unless very heavy currents are passed through the resistances. 

 The reason of this is that an electrodynamometer cannot be 

 made nearly as sensitive for small currents as a magnetic 

 galvanometer. The deflection of an electro-dynamometer is as 

 the square of the current. To make it doubly sensitive 

 requires double the number of turns in both the coils. Hence 

 we quickly reach a limit of sensitiveness. It is easy to measure 

 an alternating current of *0001 ampere and difficult for -00001 

 ampere. A telephone is more sensitive and an instrument 

 made by suspending a piece of soft iron at an angle of 45°, as 

 invented by Lord Rayleigh, is also probably more sensitive. 



For this reason I have introduced here many new methods, 

 depending upon adjusting two currents to a phase-difference of 

 90° which I believe to be a new principle. This I do by pass- 

 ing one current through the fixed and the other through the 

 suspended coil of an electrodynamometer. By this means a 

 heavy current can be passed through the fixed coils and a 

 minute current through the movable coil, thus multiplying 

 the sensitiveness possibly 1000 times over the zero current 

 method. 



I have also found that many of the methods become very 

 simple if we use mutual inductances made of wires twisted 



