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I'HE ROYAL SOCIETY OF CANADA 



dipping in a jar of sulphuric acid, through which electrical connection 

 was made with the needle. Booms suitably counterpoised, 9^', 7J^", 

 6|", and 6" long were tried in turn; the longest being used first, and 

 then the next by cutting off a piece. 



Joining one pair of quadrants to the positive terminal of an 80 

 volt battery and the other to the negative, the centre of the battery 

 being earthed, the booms gave a deflection of one millimetre for a 

 potential of 12*6; 8; 6, and 8 3 volts on the needle respectively. 

 Needles 4", 3^' and 3" were also tried with the 6^ and G" booms. 

 The faces of the quadrants were closer together for the smaller than 

 for the larger needles, but the bifiliar suspension was the same 

 in all. 



The results are given in Table I. 



TABLE I. 



Both sets of tests show that the 6^ boom and the 4^" needle 

 give the best results. Owing to the crude construction of the in- 

 struments, however, it is not possible to make any further com- 

 parison. 



If the vane is made as light as possible, the weight of the boom, its 

 counterpoise and the attachment to the needle is very much greater 

 than that of the vane of the needle, so that the sensitiveness of the 

 suspension cannot be altered appreciably for the smaller vanes, and 

 thus we get the greater sensitiveness for the larger needles. Owing 

 to the construction of the thread recorder it was not possible without 

 greatly increasing the length of the boom to make the quadrants 

 larger than would suit a 4^' needle, so that larger needles were not 

 tried. 



These considerations suggested that if a second vane were added 

 and double celled quadrants used, the weight of the suspended system 

 would not be appreciably altered, and the deflecting force on the 

 needles would be doubled, thus almost doubling the sensitiveness 

 of the instrument. 



In testing this the results given in Table II were obtained. 



