1886.] On the Self-induction of an Electric Current. 461 



infinite to its stable resistance, due to their freedom from circular 

 magnetism, whilst iron shows a comparatively slow curve. A remark- 

 able result will be seen where a copper wire has been coated with 

 iron, its variable resistance being 220 per cent, above that of its stable 

 period, and 54 per cent, greater than that of a solid iron wire. 



I found in my previous researches that the resistance in the 

 variable period could not be changed by a change in speed of a 

 periodic or tuning-fork contact maker, and that the telephone was 

 more sensitive when the rubbing contacts were used without any 

 regard to their frequency of interval. I also found that the telephone 

 was far more sensitive and alone suitable for these experiments when 

 its diaphragm was entirely free from varnish or coating, and suffi- 

 ciently thick to give a clear musical dominant tone of about C or 

 512 double vibrations per second, the electromagnet being also as 

 close as possible without being in actual contact. I noticed that no 

 matter what number of vibrations were sent, that the tones given by 

 the telephone on each side of the zero of the bridge were invariably 

 those of its dominant note. Suspecting that the frequency of vibra- 

 tion of the diaphragm had a direct relation with the resistance found, 

 I altered its note through a range of one octave by employing different 

 thickness of diaphragms, and found that the results were in absolute 

 proportion to the frequency of the vibrations of the diaphragm. The 

 highest tone giving a perfect zero with a high resistance in the 

 variable period and the low tone giving its zero at a marked less 

 resistance. Thus it Avas evident that the telephone selected from the 

 mixed vibrations sent by the transmitter those which alone cor- 

 responded to its period of vibration, and that we could thus observe 

 the effect of more or less rapid periodic contacts. 



The power which vibrating bodies have of selecting from a con- 

 fused number of vibrations those in accordance with their own, is 

 shown in Helmholtz's resonators, and still more as regards electrical 

 waves in the remarkable harmonic telegraph of Elisha Gray, who 

 proved that if the armatures of several electromagnets are made to 

 vibrate, each with a different musical tone or rate of vibration, then 

 we may by the use of a vibrating transmitter of a fixed rate of speed 

 select the electromagnet alone whose period of vibrations is similar, 

 and that we may send several different periods of vibrations at the 

 same time without any confusion in the receiving magnets, as each 

 electromagnet selects from the compound vibrations those in accord- 

 ance with its own. This is exactly what occurs with a telephone 

 when using a periodic contact maker, but I have found in addition, 

 that a telephone arranged as I have described, will select and respond 

 loudly in its own clear musical dominant tone to a confused mass of 

 rapid contacts such as those produced by a scraping contact, and 

 which in an ordinary telephone gives only the effect of noise, of not 



