﻿290 Lord Rayleigh on the Minimum Current 



The axial dimension of the section is 1*4 centim., and the 

 radial dimension is *97 centim. The mean radius A is 

 1025 centim., and the resistance is about 10 J ohms. 



In making the observations the current from the inductor- 

 coil was led to a distant part of the house by leads of doubled 

 wire, and was there connected to the telephone and resistances. 

 Among the latter was a plumbago resistance on Prof. F. J. 

 Smith's plan * of about 84,000 ohms ; but in most of the 

 experiments a resistance-box going up to 10,000 ohms was 

 employed, with the advantage of allowing the adjustment of 

 sound to be made by the observer at the telephone. The 

 attempt to hit off the least possible sound was found to be 

 very fatiguing and unsatisfactory ; and in all the results here 

 recorded the sounds were adjusted so as to be easily audible 

 after attention for a few seconds. Experiment showed that 

 the resistances could then be doubled without losing the sound, 

 although perhaps it would not be caught at once by an 

 unprepared ear. But it must not be supposed that the ob- 

 servation admits of precision, at least without greater pre- 

 cautions than could well be taken. Much depends upon the 

 state of the ear as regards fatigue, and upon freedom from 

 external disturbance. 



The pitch was determined before and after an observation 

 by removing the added resistance and comparing the loud 

 sound then heard with a harmonium. The octave thus esti- 

 mated might be a little uncertain. It was verified by listen- 

 ing to the beats of the sound from the telephone and from a 

 nearly unisonant tuning-fork, both sounds being nearly pure 

 tones. 



When the magnet was driven at full speed the frequency 

 was found to be 307, and at this pitch a series of observations 

 was made with various values of C and of R. Thus when 

 B = 7'75 inches, or C — 8*7 inches, the resistance from the 

 box required to produce the standard sound in telephone Tj 

 was 8000 ohms, so that R = 8100xl0 9 . The quantities re- 

 quired for the calculation of (5) are as follows : — 



n=242, p=2ttx307, H = «18, 



A= 10-25, ?-=8-25, d=140, 



0=8-7, R = 81xlO n , D = 1370, 



r and C being reckoned in inches, d and D in Scale-divisions 

 of about £q inch. From these data the current required to 



* Phil. Mag. rol. xxxv. p. 210 (1893). 



