504 



PROFESSOR W. E. AYRTON, MR. T. MATHER AND MR. F. E. SMITH : 



the value of the latter, and that the position for minimum rate of change of mutual 

 induction may be deduced with considerable accuracy. The force in every case is 

 approximately given by the expression : maximum force multiplied by (1 + 5'8 x lO'V/*), 

 where d is the radial displacement in mils from the coaxial position ; the corresponding 

 expression when the displacement is in centimetres is : maximum force multiplied by 

 (l + O'OOQx 3 ). Thus a displacement of 10 mils from the coaxial position produces a 

 change in force of 5'8 parts in 1,000,000. By the aid of the curves the radial setting 

 can be made within 2 mils, so that the error introduced by faulty radial setting is not 

 greater than 1 part in 5,000,000. 



DIRECTION OF DISPLACEMENT 



RADIAL DISPLACEMENT OP LEFT FI*ED CYLINDER (ARBITRARY ZEROS) 



Fig. 19. Variation of force caused by radial displacement of coils. 



Effect of the Leads. It was possible that the current in the leads to and from 

 the fixed coils might exercise an appreciable force on the suspended system, and that 

 the movable leads connected to the latter might be affected by the current in the 

 fixed system. This was tested by completing the circuit through the leads only of 

 the fixed coils and through the suspended system, and noting the effect ; afterwards 

 the current was passed through the leads only of the suspended coils and through 

 the fixed coils, and the result again noted. Absolutely no force was detectable, and 

 on a subsequent repetition of the experiment the same result was obtained. 



Insulation Tests. When making a determination of current, the greatest difference 

 of potential between any portion of the balance and the earth was about 74 volts, and 

 the greatest difference of potential between neighbouring strands on the same cylinder 

 was less than 7 volts. It is desirable, therefore, that the insulation resistance between 

 the balance circuit and earth should not be less than 100 megohms, and that between 

 neighbouring strands should not be less than 10 megohms. The insulation resistance 



