36 * Mr. J. Phillips on a Modification 



of the resin, a brass wire and ball, to which the edge of the 

 cover, or a brass ball upon it, may be applied : and this method 

 succeeds extremely well, especially with small covers which 

 can with ease and certainty be directed to any particular point 

 of the sole. The second mode is to fix a narrow strip of tin- 

 foil quite across the surface of the resinous plate, and unite it 

 at each end with the metallic basis. This construction an- 

 swers perfectly and instantaneously, and is especially conve- 

 nient with large circles, the covers of which, though uneven, 

 will thus be sure to touch some conducting point. The third 

 method of construction is to perforate the resinous plate quite 

 through to the metallic basis, at the centre, and at any other 

 points which may be thought proper, and at all those points 

 to insert brass wires, with their smoothed tops level with the 

 resin. If the surfaces of contact were perfectly plane, a cen- 

 tral wire would be sufficient, but this is seldom the case. 



To those who have not studied the electrophorus it may ap- 

 pear extraordinary that the wires or the tinfoil, at the surface 

 of the resin, do not when the cover is uplifted reduce its elec- 

 tricity to the natural state of equilibrium. They may be re- 

 minded that while the cover touches the excited electric, and 

 at the same time a body conducting to the earth, it is put into 

 a state of * forced equilibrium with the electric, by the induction 

 of that body; and that on being separated from it, and at the 

 same time from bodies conducting to the earth, the conditions 

 of this equilibrium are more and more impaired the further off 

 the cover is removed. Therefore, at very small distances from 

 the electric the cover has no sensible tendency to communi- 

 cate with any conducting body (as may be proved by electro- 

 scopes), and at greater distances, when the conditions of equi- 

 librium are proportionately diminished, the striking distance 

 of the cover is not equal to the interval between the points 

 supposed to communicate. On two of the largest electrophori 

 which I have yet made, both the second and third methods 

 have been tried with equal success, but I much prefer the lat- 

 ter construction. The largest instrument has a cast-iron basis 

 20*5 inches diameter, resinous surface 19*75 inches, cover 16*25 

 inches. — The resinous composition was made according to the 

 directions in Mr. Faraday's work on Chemical Manipulation. 

 The cover is made of a plate of thin copper, strengthened at 

 the edge by a thick brass wire, from which three radial brass 

 wires pass to the upper part of a central brass tube. In con- 

 sequence of the angle they thus form with the plane of the 

 plate, they act as pretty strong braces, to maintain its figure, 

 and the whole is very light. This central brass tube receives 

 a cylindrical piece of wood, into which the insulating glass 



