the Construction of Galvanometers, ^r. 243 



according to the greater or less predominance of magnetism in one of tl,em- 

 and the more equal the distribution of magnetism is amongst the four poles' 

 the nearer to the same vertical plane may the bars be brought without causing 

 them to swerve from the right angle at which they stand with tlie ma£,netic 

 meridian; although, when they are precisely in the same vertical plane, the needle 

 loses aU tendency to that or any otlier position. 



The condition of greatest sensibility is that in which the resultant is barely 

 so far overpowered by the predominant magnetism of one bar, that the needle 

 turns very slowly into the magnetic meridian from being at right angles with 

 It, and m passing through 180° occupies from 30 to 36 seconds 



There is sometimes great difficulty in obtaining the results described • the 

 magnetic axes may not coincide with the metrical axes, so that when the bars 

 appear to be in the same vertical plane, the magnetic axes are not so: the true 

 poles may not be equidistant from the centres of the bars: the magnetism may 

 be irregularly distributed, owing to peculiarities in the steel; and all the«e cir- 

 cumstances may combine. Empirical trials are, in such cases, tlie only resources ■ 

 and patience will insure success. ' 



The volta-magnetometer is capable of imparting sensibility to galvanometer 

 needles in two ways : first, by affording means of distributing the total quantity 

 of magneusm_ between the four poles in a degi-ee so nearly equal, that the 

 feeblest directive power only will remain, and thus the least resistance will be 

 offered to weak deflecting forces. Secondly, by causing the needle to assume 

 atrue position with regard to the magnetic meridian, and thus enabling, it to 

 give ^juM estimate of a deflecting force acting on it, which, as we have°seen 

 It does not always present. By means of this instrument, we can prepare needles' 

 of any degree of directive power, and can describe that de-jree in giving an 

 account of experiments. Results obtained by persons at a distance from each 

 other may be compared, when the sensibihty of the compound needle made use of 

 IS expressed numerically: it maybe so expressed by stating the number of de- 

 grees by which the magnetism of the two bars of the compound needle differ 

 when tested on the graduated circles of the volta-magnetometer: this will very 

 nearly give the sensibility, but absolute precision cannot be attained 



I have already described the prodess of magnetization by which the compound 

 needle acquires sufficient sensibihty for ordinary purposes; but if required to 



VOL. XXII. 2 K 



