216 MR. HARRIS ON SOME ELEMENTARY LAWS OF ELECTRICITY. 



Fig. 3. represents the wheel W with the suspended conduetor and counterpoise, the 

 index and its balance weight, together with the lines of suspension, passing freely 

 over the circumference, and fixed at the points i i. 



The various wheels above mentioned, with the graduated arc, are sustained on a 

 projecting metallic plate, which is united by a spherical nut to a metallic rod passing 

 through a glass column B. The column is secured by means of the rod to a sort of 

 double stand, h h, fig. 2, supported on three levelling screws. The interval between 

 the plates of this stand contains the glass vessel n and the micrometer screw s ; the 

 upper plate has a circular hole, /?, through which the cylindrical counterpoise passes 

 into the water, n ; the levelling screws serve to regulate the position of this counter- 

 poise through the hole p, so that when it hangs in it centrally, the instrument is ac- 

 curately adjusted. 



The gravity of the suspended conductor m being in the above arrangement op- 

 posed by that of the counterpoise, it may be so far considered as existing in free space 

 devoid of weight, and will therefore become very readily moved by any new force 

 applied to it. It may consequently be caused to approach, or recede from, the fixed 

 conductor/, by the operation of forces acting in either of these directions ; the motion 

 will however be speedily arrested by the counterpoise n, which (becoming either 

 further immersed, or otherwise raised in the water,) furnishes in the greater or less 

 quantity of water displaced, a measure of the force. In this way the force may be 

 estimated either in degrees or in grains of actual weight, since the number of grains 

 requisite to add to either side, in order to advance the index in either direction, a 

 given number of divisions may be immediately found by experiment, and which, as 

 the sections of the cylinder are all similar, will increase or decrease with the degrees 

 of the arc. Thus, if one grain advance the index in either direction five degrees, then 

 two grains will advance it ten degrees, and so on*. 



9. In the application of this instrument to electrical inquiries, the force to be mea- 

 sured is first communicated to the fixed conductor f, a free communication being 

 established between the suspended conductor m and the ground, or otherwise with 

 the negative side of the jar or battery, should the attractive force be derived from this 

 species of accumulation ; this is readily effected through the brass work of the appa- 

 ratus in connexion with the rod passing through the interior of the glass column B. 



For the repulsive force we connect the conductor y as before, and suspend m by a 

 silk thread, so as to allow it to rest on/"; it will then, after being electrified similarly 

 to /, recede from it ; but this method of experiment I have seldom resorted to ; it is 

 evidently more complicated than the former, and occasionally liable to objection. 



10. The distance between the conductors m y corresponding to a given force, is 

 easily ascertained by means of the degrees indicated on the arc x y. In the instrument 



* The counterpoise should be free from grease or varnish of any sort, and should, previously to being used, 

 be kept immersed in water ; the insulation of the conductor /, also, should be made extremely dry, and occa- 

 sionally warmed by a stick of burning charcoal. 



