520 Mr. A. 0. Allen on 



(3) The more frequent use of photography in such 

 measurements is advocated ; and 



(4) Some experimental illustrations of the photographic 

 method and of the use of the formulae of (1) and (2) have 

 been given. 



Experimental work on some of the systems mentioned 

 above is in progress. The results will, it is hoped, be of 

 sufficient interest to form the subject of future communi- 

 cations. 



University College of North Wales, 

 Bangor. 

 February, 1913. 



LA 7 I. On Measurements of Inductance. 

 By A. 0. Allen *. 



§ l.TN a report to the British Association in 1883 it was 

 JL pointed out by Lord Rayleigh that there is a marked 

 difference in the behaviour of a galvanometer used for null 

 methods of inductance measurements according to whether the 

 balance is continuous or merely aggregate; the latter case of 

 course tends to give unsteadiness. The report discusses a 

 particular case, and shows that two conditions must be fulfilled 

 if the balance is to be continuous, but no proof is given that 

 these are sufficient (as they are), and yet it might at first be 

 supposed that there would be others; for the expression 

 for the quantity passing along any arm of the network 

 consists in general of four terms, each of which must in the 

 galvanometer arm be identically equal to zero for a con- 

 tinuous balance. Moreover, no general method is indicated 

 for discovering the necessary and sufficient conditions in 

 other cases, if there are such conditions, and 1 cannot find 

 that the matter is dealt with elsewhere: at all events it is 

 not discussed in Gray's ' Absolute Measurements in Elec- 

 tricity,' which contains very full details of such work. 

 Lastly, it seems that in the cases where there is not a con- 

 tinuous balance it would be useful to know roughly the 

 duration of the flow through the galvanometer in the two 

 directions successively, so as to judge whether the instrument 

 is sufficiently ballistic. These matters form the subject of 



* Communicated by the Author. 



