350 Prof. Avrton and Mr. Mather on Galvanometers. 



zero (as depending on the controlling forces) is of prime 

 importance, the results were modified by appropriately intro- 

 ducing the moment of inertia of the suspended systems. 



Our reasons for choosing a scale-distance of 2000 scale- 

 divisions and a period of 10 seconds as standard conditions, 

 were, that these conditions represented more nearly than any 

 other round numbers those under which sensitive galvano- 

 meters were generally used. For example, scales of half- 

 millimetres used at about a metre distance were frequently 

 met with, whilst scales of fortieths of an inch used at 3 to 

 4 feet distance were quite common. Further, nearly all the 

 reflecting-instruments at the Central Technical College had 

 long scales with divisions about ^V of an inch placed at about 

 6 feet from the mirror. Since 1890, however, scales divided 

 into single millimetres have come into much more general use, 

 and may now be regarded as the standard type in physical 

 laboratories. As the present tendency in galvanometer con- 

 struction is to reduce the size of the needles and mirrors, and 

 therefore the available light, it is probable that scale-distances 

 will decrease rather than increase. We are therefore led to 

 believe that a scale-distance equal to 1000 scale-divisions 

 (rather than 2000) would be, on the whole, a more convenient 

 standard to adopt. Another reason for proposing the change 

 is to facilitate the international acceptance of a common 

 system ; for we learn from Prof. H. du Bois that our system 

 thus modified is to be recommended for adoption at the next 

 meeting of the Society of German Physicists and Physicians in 

 September next at Dusseldorf. Doubtless some instrument- 

 makers will object to the change ; others, we are sure, will 

 welcome it, for some have used the proposed standard them- 

 selves for some time. Millimetres at a metre, as the deflexion 

 per microampere, is a convenient way of stating the "figure 

 of merit' - ' of sensitive galvanometers. Moreover, the con- 

 version into circular measure is quite simple, 1 division at 

 1000 divisions distance corresponding with a movement of 

 the suspended system through ^oVo of a radian. We would 

 therefore suggest that in future all sensibilities be expressed 

 in terms of this standard. In the new sensibility-records given 

 in Table II. of the present paper we have adopted this standard ; 

 and to facilitate comparison of these records with those given 

 in our 1890 paper, we have appropriately reduced the numbers 

 given in the principal table of the latter paper, and embodied 

 them in Table I. 



As regards the standard periodic time, 10 seconds repre- 

 sented the average conditions under which sensitive galvano- 

 meters were used much more nearly than any other simple 



