472 Lord Rayleigh on the Self-induction and 



There are many cases in which we may be sure beforehand 

 that P (the effective resistance of the conductor, or combina- 

 tion of conductors, to the variable currents) is the same as if 

 the currents were steady, and then P may be regarded as 

 known. There are -other cases, however, — some of them will 

 be treated below, — in which this assumption cannot be made ; 

 and it is impossible to determine the unknown quantities L 

 and P from (2) alone. We may now fall back upon (1). By 

 means of the two equations, P and L can always be found in 

 terms of the other quantities. But among these is included 

 the frequency of vibration ; so that the method is only prac- 

 tically applicable when the interrupter is such as to give an 

 absolute periodicity. A scraping contact, otherwise very 

 convenient, is thus excluded*; and this is undoubtedly an 

 objection to the method. 



My own experiments have been made with three different 

 forms of apparatus. The first was constructed upon the model 

 of that originally described by Hughes, and still to be pre- 

 ferred for some purposes. The others will be described in due 

 course ; but it will be convenient to consider first those parts 

 which are common — the interrupters and the induction- 

 compensators. 



The Interrupters. 



When regular vibrations are not required, a scraping con- 

 tact interrupter is the least troublesome. Mine is of the 

 roughest possible -construction. It is driven by a small jet of 

 water issuing from a glass nozzle in communication with a 

 tap, and impinging upon blades bent in a piece of tin plate 

 and revolving about a vertical axis. The upper part of the 

 axis carries a small cylinder of roughened iron, against which 

 a brass spring lightly presses. As in Hughes's apparatus, the 

 scraping contact is periodically broken altogether by a pro- 

 jecting finger, which during part of the revolution pushes 

 back the brass spring. This is a point of some importance, 

 for a faint scraping sound is far better heard and identified 

 when thus rendered intermittent. The apparatus stands in 

 the sink, so that the water scattered from the revolving blades 

 runs away without giving trouble. The pressure exercised 

 by the contact-spring requires readjustment from day to day 

 if the loudest sound is wanted. 



But for many of the most interesting experiments a scraping 

 contact is unsuitable. Prof. Hughes has found, indeed, that 



* A topthed- wheel interrupter, as usually employed, does not give a 

 regular vibration of the period corresponding to the passage of a tooth. 



