60 Messrs. Ayrton, Mather, and Sumpner 



current with the non-astatic galvanometer, may be small for a 



given value of the deflexion a, the resultant controlling field 



F must be small; whereas with the astatic combination there 



is no necessity to make F small in order that C 2 may be small, 



M 



since ^ may be made large. Now the disturbance that an 



M— r/i 



extraneous magnetic body can bring about depends on the 

 strength of the magnetic field it sets up compared with that 

 of the field F produced by the earth and the adjusting mag- 

 net combined. Hence the moving about of a magnetic body 

 in the neighbourhood of the galvanometer will produce more 

 disturbance on a non-astatic galvanometer than on an astatic 

 galvanometer of the same sensibility. The preceding are 

 very important considerations ; hence, so far from agreeing with 

 Prof. Gray that it is more troublesome to work with astatic 

 galvanometers than with non-astatic ones, the preceding con- 

 siderations, which we find fully borne out by experience, have 

 led us to regularly employ the astatic principle with sensitive 

 galvanometers. 



And generally we may conclude that when it is desired to 

 utilize a magnetic control, for example, with electrometers, 

 &c, it is better, when a small controlling moment is required, 

 to obtain this by the use of a weak magnet on the suspended 

 system than by attempting to simply weaken the controlling 

 field. Indeed, we may mention that even in the case of elec- 

 trometers, where there is no question of the action of currents 

 on magnets, and where (when a magnetic control has been em- 

 ployed in place of a bifilar or torsional control) it has been 

 usual to attach a single little magnet to the aluminium elec- 

 trometer-needle, to be acted on by a stationary outside magnet, 

 we have found it more convenient to attach an astatic com- 

 bination of magnets to the moving needle, and to direct the 

 system by a fairly strong permanent magnet, which acts of 

 course differentially on the suspended astatic system of mag- 

 nets, since with such an arrangement great freedom from out- 

 side magnetic disturbance is secured, combined with the 

 power of employing a wide range of sensibility. 



It is worth noticing that in galvanometers, where the dis- 

 tance of the controlling magnet from the needles is varied by 

 simply raising the magnet M, as in the case of the galvano- 

 meter seen in fig. 1, it is more convenient to place the 

 galvanometer so that the plane of the coils is in the magnetic 

 meridian of the laboratory when the controlling magnet is 

 removed; or, in other words, to place the galvanometer so that 

 the spot of light comes to the zero of the scale for no current 



