Electricity and Galvanism. 251 



The electrical machine consists of a revolving cylinder, or plate of 

 glass, submitted to the friction of cushions, or rubbers. It matters 

 very little what form of machine is employed. As a general rule, a 

 plate machine is, for equal size, of far higher power than the cylinder. 

 The arrangements of the latter are, however, simple, and are, perhaps, 

 more easily managed by the uninitiated. There is also an advantage 

 on the score of economy, as old cylindric machines are readily to be 

 procured at low prices, and, as a general rule, a well-worn cylinder is 

 far preferable to a new one. Plate machines are, on the contrary, 

 less common, and consequently must generally be purchased new. 

 Whichever form is employed, it is useless using a plate with a less 

 diameter than a foot, or a cylinder less than five or six inches. 



There is some little tact required to elicit the full power of an electric 

 machine, and, from want of this, you will frequently find some persons 

 quite fail in exciting any amount of electricity even from the best con- 

 structed machines. This art is, however, soon acquired. When the 

 machine is required for use, the prime conductor and rubbers should 

 first be removed, and the machine placed sufficiently near a good fire 

 to become completely dry and warm. The surface of the glass should 

 then be slightly rubbed with a piece of tow or flannel soaked in olive- 

 oil, any adhering black spots from old amalgam being scraped off. 

 By means of a dry and warm linen cloth, the oil should then be 

 wiped away, and the polished surface of the glass is thus left clean 

 and free from moisture. The cushions, if covered with amalgam, 

 are then to be rubbed with a piece of brown paper, so as gently to 

 remove the oxydized surface ; but if not sufficiently covered, a little 

 amalgam (made by melting together zinc two parts, tin one part, with 

 mercury six parts, made into a paste by triturating it in a mortar 

 with a little lard) must be rubbed into the surface of the cushions 

 with the handle of a knife, or a piece of smooth wood. The silk flaps 

 are to be wiped clean, and the rubbers adjusted to the plate or cylin- 

 der. On revolving the latter, a rustling noise will be heard, accom- 

 panied, in a darkened room, by vivid flashes of blue light, whilst a 

 strong phosphorus-like odour of ozone becomes perceptible. The 

 prime conductor is next to be replaced, taking care that its insulating 

 support is perfectly dry, and even slightly warm : the instrument is 

 then fit for use. You will, however, not unfrequently find, that 



