﻿112 Dr. W. H. Eccles on the Effect of Electrical 



small magnets ; readings were taken on a scale distant 

 88 cm. 



The object in employing twin solenoids is clear. By 

 placing equal amounts of iron wire in each solenoid and 

 adjusting their positions carefully, the magnetometer deflexion 

 could be kept very slight whatever magnetic variations the 

 iron was taken through, the iron in the one solenoid 

 compensating that in the other. 



It was then permissible to exalt greatly the sensibility of 

 the magnetometer. This was done by reducing the controlling 

 field at the needle by means of an auxiliary permanent magnet. 

 As a matter of fact, slight inequalities existing in the con- 

 struction of the solenoids made perfect balance unattainable ; 

 there was always a remanent deflexion, which was different in 

 sign and magnitude for different values of the current through 

 the solenoids, and which in the main proved rather useful by 

 furnishing a check on the proper carrying out of the 

 commutating operations involved in a magnetic cycle. 



In the experiments, three iron wires insulated from one 

 another and tied in a bundle were used in each solenoid, the 

 group in the east coil lying, of course, in the glass tube on 

 which the oscillation-coil was wound. The inner ends of the 

 wires were 9*3 cm. distant from the magnetometer needle. 

 The field at the needle was reduced to 0*034 C.G.S. units. 

 With these arrangements, such a sensibility was attained that 

 on certain days of August last the strokes of distant lightning, 

 not visible in London *, were easily perceptible in the 

 laboratory by their effects on the iron in the oscillation-coil. 



The oscillations used were produced by leading the free 

 east end of the oscillation-coij, that is the end distant from 

 the magnetometer, to one side of a micrometer spark-gap. 

 The other end of the oscillation-coil was left insulated. The 

 poles of the spark-gap were connected to the terminals of a 

 diminutive influence machine, that pole not connected to the 

 oscillation-coil being, besides, earthed. Thus, when the 

 handle of the influence machine was turned through a certain 

 angle (depending on the spark-gap) at a speed easily learned, 

 a spark occurred which set up oscillations in the coil. The 

 spark-length finally settled upon was about half a millimetre. 

 It was found possible in this way to get over and over again 

 practically the same magnetometer deflexion for every spark, 

 provided the effect of previous oscillations was wiped out by 

 taking the iron through a cycle. As these small sparks were 

 usually inaudible, their occurrence was recognized by the 

 sudden deflexion of the magnetometer-needle. 



* The newspapers gave accounts of thunder-storms in Hertfordshire. 



