298 Mr. J. N. Hearder on Electrical Machines. 



faces, and whatever their construction, their effects are equivalent 

 to the number of rubbers employed. In proof of this I may 

 quote the following. An excellent double 24-inch plate machine 

 with eight rubbers, each 5^ inches long, and rubbing at each 

 revolution rather less than 18^ feet, requires, when in good ex- 

 citation, five turns, or 91 square feet to produce a given dis- 

 charge. 



An excellent 3-feet plate machine, the property of Sir W. S. 

 Harris, fitted with four rubbers, each 10 inches long, rubs about 

 24< square feet at each revolution, and when in good condition 

 requires the friction of 90 square feet to produce the same effect. 



I have at various times compared these machines with each 

 other, and with smaller machines, all excited with the same 

 amalgam, and find a remarkable uniformity of result. 



It now remains to com])are plates with cylinder machines. 

 On one occasion I compared a 7^-inch cylinder, rubbing 176'25 

 square inches at each revolution, with Sir W. S. Harris's 3-feet 

 ])late machine; both were in excellent condition, but not excited 

 with the same amalgSm. A standard jar was employed with a 

 Lane's discharger, and a striking interval of 0"5 of an inch. The 

 quantity of surface rubbed by the cylinder to produce the dis- 

 charge was about fifteen turns, or 18'33 square feet, whilst the 

 plate machine required 3| tvu-ns to produce the same effect. As 

 a single rubber rubbed about 6 square feet, the whole surface 

 would be 6x4x3| = 90 square feet, or about five times the 

 surface rubbed by the cylinder. 



Again, the same cylinder was compared with a double 24-inch 

 plate machine in my own possession, every care being taken to 

 use the same amalgam and maintain the same conditions. In 

 this case the quantity of surface rubbed upon the plate was 

 rather more than four times as much as that on the cylinder. 



Thirdly, a 9f -inch cylinder with an 8-inch rubber, rubbing 

 244 square inches at each turn, produced a given discharge with 

 a friction of 41 '36 feet, whilst the double plate machine used at 

 the same time and excited with the same amalgam, required 165 

 feet to produce the same effect. 



It thus appears that cylinder machines have a great supe- 

 riority in their exciting power over plate machines, and for ordi- 

 nary expenmental purposes are much more convenient. A 

 cylinder of 12 inches diameter, having a single ru,bber of 9 inches 

 in length, will be just equal to a 24-inch plate machine having 

 four rubbers, each 5i inches long; or if the cylinder be fitted with 

 two rubbers as befox'e described, it will be equal to a double plate 

 machine which possesses the disadvantage of having eight rub- 

 bers to keep in order. In very large plate machines, where a 

 single pail" of rubbers only is used, the sacrifice of power in pro- 



