510 Measurement of Small Capacities and Inductances. 



every alternate inductive electromotive force due to the 

 inductance of the coil, and cause the galvanometer to give a 

 steady deflexion. To evaluate this inductance in absolute 

 measure, we prefer the method described by Professor A. 

 Anderson *. In this method a variable resistance r is placed 

 in the bridge circuit in series with the galvanometer con- 

 nected to a Wheatstone-bridge, and a condenser of known 

 capacity is joined in, as shown in the diagram (see fig. 7), 

 and the double inter raptor inserted in the battery and gal- 

 vanometer circuits. When the steady or ohmic resistance of 

 the inductive coil has been determined in the usual manner, 

 the interruptor is set in rotation, and the value of the resist- 

 ance r in series with the galvanometer is altered until the 

 galvanometer deflexion vanishes. This last change does not 

 upset the adjustment of the arms of the bridge already made 

 in obtaining the ohmic resistance of the coil. If P. Q, S are 

 the arms of the bridge, and E is the ohmic resistance of the 

 coil under test, of which the inductance is L, then the ohmic 

 resistance R and inductance L of the coil are given by the 

 equation 



R = PS/Q, 



L = CWR + S)+RQ}t. 



Fig. 7 shows the galvanometer and battery connected to a 

 change-over switch m m in order that they may be placed 

 relatively to the four arms of the bridge in the position 

 giving the most sensitive arrangement for the resistances 

 employed. We understand that Prof. Stroud has employed 

 one of these arrangements for some time past in the measure- 

 ment of inductance, but no publication of it appears to have 

 been made prior to that by Prof. Anderson. 



This method of measuring inductance by a combination of 

 the double interruptor, with the Anderson method, is one of 

 the most perfectly satisfactory methods that can be employed 

 in the laboratory. It is an absolute method in that it 

 requires no arbitrary standard of inductance, and yet, at 

 the same time, it requires no determination of a speed ; it 

 only assumes the possession of resistance-boxes and a known 

 capacity. 



The value of small inductances, such as are best reckoned 

 in millihenrys, can be obtained most accurately by measuring 

 them as the difference of two larger inductances. Thus, for 



*See Phil, Mag. vol. xxxi. p. 329 (1891) ; or 'The Electrician,' vol. 

 xxvii. p. 10. 



f For the proof of this formula for the inductance, we refer to the 

 original paper by Professor A. Anderson (loc. cit.). 



