by equal surfaces of Cylinder and Plate Electrical Machines. 297 



hour, or until reduced to a fine black powder, and preserved for 

 use in this form. When used, it is ground up very fine with a 

 small quantity of fresh lard, and laid upon the rubber as smoothly 

 as possible. With this amalgam, a well-constructed machine, 

 having a cylinder of 32 inches in circumference, and a rubber 

 9 inches in length, with a prime conductor 4 or 5 inches dia- 

 meter and 20 inches long, should, without the necessity of 

 warming the cylinder, give from four to six straight dense sparks 

 of 3 to 3|^ inches in length, for every revolution of the cylinder. 



A 2-inch ball inserted into the conductor should throw off 

 spontaneously rapid brushes of electricity into the aii', and fur- 

 nish zigzag sparks of 9 inches or more in length. It should 

 also charge a Leyden jar of ordinary thickness containing 4 

 square feet of internal surface, so as to discharge at 0'5 of an 

 inch with 48 or 50 turns. 



Whatever the size of the machine, excepting perhaps those of 

 very small dimensions, it may be generally assumed that the 

 friction of about 24 square feet should be sufficient to charge a 

 square foot of coated glass to the intensity of 0*5 of an inch. 



On comparing these effects with those obtained from a plate 

 machine, we observe an extraordinary difference in relation to 

 the quantity of surface submitted to friction, since the plate ma- 

 chine scarcely yields with four rubbers as much electricity as it 

 might be expected to develope with one. I have been at some 

 pains to analyse the action of the plate machine, and I find, that, 

 with an exceedingly small allowance for the effect of a silk flap 

 without a rubber, the power of the machine is, as nearly as pos- 

 sible, equal to the sum of the effects of the rubbers singly, and 

 that it is immaterial, when both surfaces of the plate are rubbed, 

 whether the conductor collect the electricity from both surfaces 

 or only from one. When a single rubber, however, is applied, 

 there is a slight advantage in collecting the electricity from the 

 excited side of the plate in the proportion of 19 to 18, though 

 the quantity developed from a single rubber is only about one- 

 fourth of that produced by the friction of an equal surface on a 

 cylinder. That the conductor should continue to collect elec- 

 tricity from the surface of the plate which is not rubbed, without 

 accumulating a charge on the rubbed surface, is an anomaly 

 which I am unable to explain, except on the assumption that 

 electrified surfaces conduct electricity, and that it passes round 

 from one side to the other. 



liy a comparison which I have made of the power of plate 

 electrical machines from the diameter of 18 inches, to that of 

 tlic large jilatc inachine at the Polytechnic Institution, I find 

 that, when in good excitation, they are pretty nearly alike as to 

 the quantity of electricity developed by the friction of equal sur- 



