Resistance of Compound Conductors. 473 



in some cases the natural pitch of the telephone-plate is pre- 

 dominant ; so that the vibration, as it reaches the ear, is not 

 quite so mixed as might have been expected from its origin. 

 When, however, the induction and resistance under observa- 

 tion are rapidly varying functions of the frequency of vibra- 

 tion, it is evident that no sharp results can be obtained 

 without an interrupter giving a perfectly regular electrical 

 vibration. With proper appliances an absolute silence, or at 

 least one disturbed only by a slight sensation of the octave of 

 the principal tone, can be obtained under circumstances where 

 a scraping contact would admit of no approach to a balance 

 at all. 



A thoroughly satisfactory interrupter of this kind has not, 

 to my knowledge, been constructed. Tuning-forks, driven 

 electromagnetically with liquid or solid contacts, answer 

 well so long as the frequency required does not exceed 128 or 

 256 per second ; but here we desire frequencies of from 500 

 to 2000. My experiments have been made with harmonium- 

 reeds as interrupters, the vibrating tongue making contact 

 once during each period with the slightly rounded end of a 

 brass or iron wire, which can be advanced exactly to the 

 required position by means of a screw cut upon it. Blown 

 with a well regulated wind, such reeds have given good 

 results even up to 2000 (complete) vibrations per second ; 

 but they are often capricious and demand frequent readjust- 

 ment. The reed which I have usually employed makes 

 about 1050 vibrations per second, and answered its purpose 

 fairly well. Hitherto I have not been able to satisfy myself 

 as to the cause of the falling off in efficiency, which often sets 

 in suddenly, and persists until cured by a readjustment. 

 Another objection to this interrupter is the simultaneous pro- 

 duction of loud aerial sounds, which must be prevented from 

 reaching the ear of the observer at the telephone by several 

 interposed doors. 



The Induction- Compensators. 



Two instruments, similar in all respects, were made by my 

 assistant Mr. Gordon, much after the pattern employed by 

 Prof. Hughes. In each there is a small coil mounted so that 

 one diameter coincides with a diameter of a larger coil, and 

 movable about that diameter. The mutual induction M 

 between the two circuits depends upon the position given to 

 the smaller coil, which is read off by a pointer attached to it 

 and moving over a graduated circle. The circles are so 

 divided that the reading (6) would * be zero when the axes 

 * The position is mechanically unattainable. 



