CHAP, i.] ELECTRICITY AND MAGNETISM. 45 



surface of the rubbers. An amalgam of zinc is also 

 effective ; though still better is Kienmayer's amalgam, 

 consisting of equal parts of tin and zinc, mixed while 

 molten with twice their weight of mercury. Bisulphide 

 of tin (" mosaic gold ") may also be used. These 

 amalgams are applied to the cushions with a little stiff 

 grease. They serve the double purpose of conducting 

 away the negative charge separated upon the rubber 

 during the action of the machine, and of affording as a 

 rubber a substance which is more powerfully negative 

 (see list in Art. 5) than the leather or the silk of the 

 cushion itself. 



42. Precautions in using Electrical Machines. 

 Several precautions must be observed in the use of 

 electrical machines. Damp and dust must be scrupu- 

 lously avoided. The surface of glass is hygroscopic, 

 hence, except in the driest climates, it is necessary to 

 warm the glass surfaces and rubbers to dissipate the 

 film of moisture which collects. Glass stems for in- 

 sulation may be varnished with a thin coat of shellac 

 varnish, or with paraffin (solid). A few drops of 

 anhydrous paraffin (obtained by dropping a lump of 

 sodium into a bottle of paraffin oil), applied with a bit of 

 flannel to the previously warmed surfaces, hinders the 

 deposit of moisture. An electrical machine which has 

 not been used for some months will require a fresh coat 

 of amalgam on its rubbers. These should be cleaned 

 and warmed, a thin uniform layer of tallow or other stiff 

 grease is spread upon them, and the amalgam, previously 

 reduced to a fine powder, is sifted over the surface. 



All points should be avoided in apparatus for 

 frictional electricity except where they are desired, like 

 the " collecting " spikes on the prime conductor, to let off 

 a charge of electricity. All the rods, etc., in frictional 

 apparatus are therefore made with knobs, so as to avoid 

 sharp edges and points. 



43. Experiments with the Electrical machine. 



rY 



