376 Prof. Ayrton and Mr. Mather on Galvanometers. 



To put the matter in another way, we may determine the 

 relative times after which the spots will have moved appre- 

 ciably from the zero. A little consideration will show that 

 these times will depend on the relation of the least visible 

 movement to the maximum displacement produced by the 

 want of balance it is necessary to detect. For example, if 

 the sensitiveness of the short-period instrument is such as to 

 give a fairly large movement of the spot for the given want 

 of balance, then a perceptible movement will occur in a time 

 which is a small fraction of its period, and this motion will 

 take place in practically the same time as an equal motion of 

 the spot of the long-period instrument. On the other hand, 

 if the sensitiveness of the short-period instrument is only 

 just sufficient to show the given want of balance when the 

 spot reaches its maximum elongation, then half a period 

 must elapse before the want of balance would be detected, 

 and the more sensitive long-period instrument would have 

 moved an equal distance in a shorter time. In this case, 

 therefore, the more sensitive instrument would have a distinct 

 advantage in quickness in showing the want of balance, but 

 more time would be required for its return to zero before 

 another test could be made. 



To show the relative times under different conditions of 

 required accuracy, and of different relative periods of similar 

 galvanometers, we have calculated the following table : — 



Table YIII. 



Eatio of maximum dis- 

 placement of short - 

 period needle to least 

 movement visible. 



T 



ime taken by short-period galv. to move visibly 



longer „ 

 when the sensitiveness of the longer-period instrument is 

 m 2 times as great as that of the short-period instrument. 



| 



I 





m 2 = 2. 1 m 2 =4. 



;;r=16. 



w 2 = 64. \m 2 = 10,000. 



1 



1-41 



1-50 



1-55 



1-60 



163 



2 



1-06 



1-09 



140 



110 



1-11 



3 



1-034 



1-05 



1-053 



1-054 



1-055 



4 



1025 



104 



1045 



1047 



1-048 



8 



1011 









1021 



10 



101 



... 





... 



1-019 



20 



1005 



■ 





1-007 



i 



The first column gives a measure of the sensitiveness of 

 the short-period instrument in terms of the minimum possible 



