290 



Messrs. T. Gray and A. Gray. [Feb. 14, 



of about 120 Daniells to measure resistances of from 10 4 to 10 5 

 megohms. 



To increase the rapidity with which observations can be taken, the 

 instrument can be fitted with a vane immersed in a liquid ; or placed 

 in a nearly closed air-chamber of proper dimensions. 



In another form of the instrument (fig. 2), of which an example is 

 being completed for Sir William Thomson for use as a ballistic 

 instrument, the four coils are arranged symmetrically in one plane, 

 and a horizontal curved (slightly S or Z shaped) bar or frame of 

 aluminium used to carry the vertical horseshoes. This frame passes 

 through from one side to the other of the plate carrying the coils, so 

 that one horseshoe enters its pair of coils from one side, and the other 

 horseshoe its pair of coils from the other side. The coils are joined 

 so that when a current is sent through the instrument both horseshoes 

 are dragged further into their coils, or both pushed out at the same 

 time. The needle system is thus turned round the suspension fibre, and 

 the deflection is measured by means of a mirror and scale as before. 



This form of the instrument has the advantage of being more 

 compact and more easily constructed, and of admitting of readier 

 adjustment of the magnets relatively to the hollow cores of the 

 bobbins. The cores are made somewhat " trumpet- shaped " where 

 the needles enter. Of course as the instrument which is being made 

 is to be used for ballistic experiments, the cores have not been 

 formed of metal tubes as in the instrument already described. 



The chief advantage of the arrangement of coils and needles 

 described above is that a great portion of the wire of the coils is 

 placed very near to the poles of the needles, and in a very favourable 

 position for exerting the electro-magnetic action required. The 

 instrument, particularly the form shown in fig. 2, is very easily made, 

 and does not cost more than an instrument of the ordinary kind. Of 

 course a single horseshoe, or S or Z shaped bar, might be placed 

 horizontally, and acted on by a pair of coils, and the principle thus 

 applied to a single needle non-astatic instrument. In astatic instru- 

 ments, however, of this form it is decidedly preferable, as shown 

 below, to use vertical needles. 



It is to be observed that if the line joining the poles or centres of 

 gravity of magnetic polarity in each horseshoe be vertical, the system 

 is perfectly astatic for a uniform field, for each vertical horseshoe is 

 itself perfectly astatic. The pair of horseshoe needles can thus be 

 adjusted to have as nearly as may be perfect astaticism in a uniform 

 field, and thus made to preserve a nearly constant zero when under 

 directive force, a result which it is exceedingly difficult to obtain in 

 the ordinary arrangement of horizontal needles, and which certainly 

 rarely exists when a horizontal magnet or magnets placed above or in 

 an unsymmetrical position relatively to the needles is employed to 



