and the Theory of the Induction-balance, 125 



Faraday interposed blocks of copper between a primary coil 

 and a secondary connected with a galvanometer, and was sur- 

 prised to find that the effect at make and break was precisely 

 the same with the copper as without it*. It was, however, 

 afterwards found that the physiological effects were very dif- 

 ferent, being much less when the copper was present — thus 

 proving that though the copper did not affect the integral flow 

 of electricity, yet it greatly affected the time during which 

 that flow took place. 



Dove repeated Faraday's experiment more perfectly by 

 means of an induction-balance, and showed that no non-mag- 

 netic media produced any effect appreciable by a galvano- 

 meter. 



Thus may numerous phenomena be overlooked with a gal- 

 vanometer which with a telephone become obtrusively evident. 

 2. My attention was more particularly directed to the sub- 

 ject by an observation which was made by Mr. W. Grant, 

 assistant in the Physical Laboratory, University College, and 

 which I have his permission to describe. 



The intermittent current from a mouth-telephone, or the 

 induced current from a clock-ticking microphone and coil, 

 is sent through a long helix of wire wound upon a bobbin, 

 with another similar but quite disconnected wire wound along- 

 side it. A telephone and ear are also arranged in the cir- 

 cuit of the first wire, and the loudness of the sound observed. 

 The disconnected wire, which is wound on the same reel as 

 the first, now has its two ends joined up so that it itself forms 

 a closed circuit : the loudness of the sound is thereby consi- 

 derably increased. The secondary circuit is broken again, and 

 the sound again becomes faint. The strengthening on closing 

 the secondary circuit is so great, indeed, that short-circuiting 

 the primary so as to shunt out all its resistance, scarcely pro- 

 duces any additional strengthening effect. In other w T ords, a 

 moderate resistance (several ohms) may be thrown into a 

 telephonic circuit without the slightest appreciable weakening 

 of the effect, provided a second wire coiled alongside the first 

 be arranged so as to form a closed circuit. If a second tele- 

 phone be put in circuit with this second wire, it will give 

 about the same indications as the other ; and this may some- 

 times be a good way of working two telephones. 



The rough general explanation is, of course, not far to seek. 

 The extra currents, which at first oppose the primary, are 

 able to form in the secondary when its circuit is closed ; and 

 hence the changes in strength of the primary are more rapid, 

 and therefore more complete. 



* Exp. Ees. arts. 1721, 1735. 



