24 THE ROYAL SOCIETY OF CANADA 



the negative, the centre being earthed. A copy of a record of the 

 potential is given in Fig. V. The record was obtained when an area 

 of low pressure was approaching, giving cloudy weather. 



Zero. 



Hour Marks 



Figure V 



Conclusion. 



The question naturally arises whether it is better to make a needle 

 with two vanes rather than an equivalent single vaned needle. 



If one increased the sensitiveness by increasing the size of the 

 needle, the best that it would be possible to do, would be to make an 

 instrument that had its plates the same distance apart for the larger 

 as for the smaller needle and that had the needles constructed of ma- 

 terial of the same gauge. In this case, if the linear dimension of the 

 needle were multiplied by u, the capacity would be multiplied by u^. 

 The surface density for the same voltage would not change, nor the 

 electro mechanical force per unit area, nor the moment about the 

 suspending wire of the electric force per unit area. The total moment 

 would then be increased by u^, or the deflecting force on the needle 

 would be multiplied by the increase of area of the needle. Thus if 

 the area of the needle were doubled the weight would be doubled, 

 and the deflecting force doubled. This is the best that it would be 

 possible to obtain. If it is assumed that it is possible to obtain this 

 condition, would it give an instrument as sensitive as one obtained 

 by adding two vanes and using double celled quadrants. Almost the 

 whole weight of the suspended system is in the boom, and its counter- 

 poise and the weight of the vane can be neglected so that by doubling 

 the area of the needle the deflecting force would be doubled also, pro- 



