by equal surfaces of Cylinder and Plate Electrical Machines. 293 



experiments with flaps of various materials^ including single and 

 double silk, plain and varnished with shell-lac, oil, caoutchouc, 

 and other materials, it will be sufficient to state the kind which 

 gives the greatest effect. After more than thirty years' experi- 

 ence with some scores of machines, I have found that the thin yel- 

 low oiled silk makes the best flap for an electrical machine, when 

 properly prepared for the purpose. This material is best var- 

 nished on one side only, the unvarnished side being applied to the 

 glass, and requiring no preparation. If the oiled silk be var- 

 nished on both sides, it adheres so closely to the glass as to ren- 

 der it impossible to turn the machine, and it therefore requires 

 to be prepared by giving it two or three coats of shell-lac varnish 

 on the side to be applied to the glass, which will effectually pre- 

 vent the too strong adhesion. A new flap of this kind improves 

 much in action after the machine has been excited three or four 

 times, evidently showing the gradual attainment of qualities not 

 possessed at first, and moreover proving that the flap itself is a 

 necessary element in the excitation. 



The arrangement of the rubber also is a matter of no small 

 importance. It is absolutely necessary that there shall be a per- 

 fect contact of the cylinder with the rubber and silk flap, from 

 the face upon which the amalgam is spread to the end where the 

 prime conductor receives the electricity; hence the superiority of 

 oiled silk, the thinness of which enables it to adapt itself to the 

 irregularities of the cylinder. 



As a proof of the foregoing remarks, it is merely necessary to 

 try the following experiment. 



When the machine is in tolerable action, pass under the silk flap 

 a piece of silk cord, in a direction parallel to the rubber, and at 

 an inch above it, and hold it by the ends so as to touch the 

 cylinder. The silk flap will thus be kept from touching the 

 cylinder in the part where the silk cord is strained across, and this 

 slight separation will be found considerably to impair the action 

 of the machine. The stouter the cord, and the nearer it is held 

 to the rubber, the greater is its influence, the loss of power 

 amounting to one-half or three- fourths of the whole; hence 

 the practice of sewing the silk flap on to the upper edge of the 

 rubber is most objectionable. 



The best plan is to attach it to the lower edge of the rubber, 

 and allow it to pass up between the rubber and cylinder. In 

 order to prevent the injury which would accrue to the flap from 

 repeatedly spreading the amalgam upon the surface which covers 

 the rubber, a second piece of silk, which is better not oiled, is 

 attached to the lower edge of the rubber, and turned up over its 

 face, so as to receive the amalgam. When torn, it can be replaced 

 by another piece. 



