524 



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



of one arm of the balance is 25'4 centims., and the " equivalent arm " of each pair of 

 strips is about 15 centims. ; hence the maximum downward force on the strips was 

 65 dynes, equal to that produced by 66 milligrammes. It is interesting to note that 

 the mass of the two strips was less than this being only 36 milligrammes. The 

 elasticity of silver changes with temperature and the control exercised by the strips 

 must in consequence have varied with it ; calculation shows, however, that the effect 

 was negligible. 



To remedy these defects, each of the four strips was replaced by 80 silver wires 

 1 mil (0'0025 centim.) in diameter. The surface per centimetre length of the strip 

 was 0'80 sq. millim. and the section of the strip 0'013 sq. millim. ; the corresponding 

 values for the 80 strands are 6'0 sq. millims. and 0'04 sq. millim. The length of each 

 strand is 10'5 ceutims., and the resistance of the 80 is about 0'037 ohm ; the heating 

 effect is, therefore, one-quarter of that formerly experienced, and the radiating 

 surface over seven times as great. The sensitiveness of the balance is greater than 

 when the strips were used, and the current through the fine wires can be kept very 

 constant. In addition there is no drift in the resting point of the balance due to 

 convection currents of air rising from the silver wires. Fusion of the silver did not 

 result when a current of 07 ampere was passed through one strand. 



(C) Heating Effect of Current in. Balance Coils. The total resistance of the fixed 

 and suspended coils is 71 ohms at 17 C. With a current of 1 ampere the heating 

 effect is considerable and the resistance of the coils changes comparatively rapidly. 

 The following table (XI.) gives the resistance of the balance coils and estimated 

 temperatures when currents of 070 and 1*02 amperes respectively pass through the 

 coils until the latter are in a steady thermal state. In each case the circuit was 

 completed for 24 hours. 



TABLE XL 



Afterwards the balance case was covered with blankets and similar observations 

 made with a current of 1 ampere. The maximum increase in temperature was 

 22 C., the temperature of the air within the balance case being 12 C. lower than 

 that of the suspended coils. 



