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XLVII. On the Conjugate Positions of two Circular Coils 

 of Wire. By W. Grant, Assistant in the Physical Labo- 

 ratory, University College, London* . 

 [Plate XII.] 



WHILE recently engaged on some experiments on in- 

 duction, I observed certain circumstances which I 

 had not before noticed, and which seemed deserving of further 

 attention. I was therefore led to inquire a little more closely 

 into these matters ; and although the investigation is by no 

 means full or complete, I have obtained one or two results 

 which I thought I might venture to lay before the Physical 

 Society. The apparatus used in these experiments consisted, 

 as at first arranged, of two coils of copper wire, one of which 

 was connected in circuit with a battery of three Leclanche 

 cells, and with a microphone which was actuated by a watch, 

 while the other was connected with a telephone, in order that 

 the induced currents, while passing through it, might render 

 audible the beating of the watch which was used as the source 

 of sound. 



A modification of this arrangement was afterwards tried, 

 a Grove's battery of twelve cells being substituted for the 

 Leclanche battery, and a key being used for making and 

 breaking the circuit. This was done in order to obtain 

 greater inductive effects between the coils than could be ob- 

 tained from the variations in the strength of the current which 

 were caused by the action of the microphone. It was found, 

 however, that with a little care in the adjustment of the coils, 

 one cell gave sensibly as great an effect in the telephone as 

 twelve cells ; in subsequent experiments, therefore, the Grove's 

 battery was discarded, and that of Leclanche again re- 

 sorted to. 



Now if two similar coils, connected as above described, 

 are arranged with their planes parallel and their axes coin- 

 cident, it is found that they may be separated to a considerable 

 distance before the sounds which are heard in the telephone on 

 making and breaking the circuit are obliterated. But it is 

 also found that if the planes of the coils are kept parallel, the 

 one in connexion with the telephone (that is, the secondary 

 coil) may be placed in certain positions in the neighbourhood 

 of the primary coil, and even in contact with it, without 

 sounds being heard in the telephone. This happens when 

 the mutual inductive effect between the two coils becomes 

 zero ; and when they are so placed as to fulfil this condition, 

 tehy are said to occupy conjugate positions relatively to each 

 other. 



* Communicated by the Physical Society, having: been read June 28th, 

 1*79. 6 



