intervals.  The slips at opposite ends of a diameter are connected together twice during each revolution of the plates by wire brushes S, and collecting combs TT serve to charge the positive and negative conductors cc, which yield very powerful sparks at the knobs K above.  The given theory of this machine may be open to question, but there can be no doubt of its wonderful performance.  A small one produces a violent spark 8 or 10 inches long after a few turns of the handle.

The electricity of friction is so unmanageable that it has not been applied in practice to any great extent.  In 1753 Mr. Charles Morrison, of Greenock, published the first plan of an electric telegraph in the Scots Magazine, and proposed to charge an insulated wire at the near end so as to make it attract printed letters of the alphabet at the far end.  Sir Francis Ronalds also invented a telegraph actuated by this kind of electricity, but neither of these came into use.  Morrison, an obscure genius, was before his age, and Ronalds was politely informed by the Government of his day that “telegraphs of any kind were wholly unnecessary.”  Little instruments for lighting gas by means of the spark are, however, made, and the noxious fumes of chemical and lead works are condensed and laid by the discharge from the Wimshurst machine.  The electricity shed in the air causes the dust and smoke to adhere by induction and settle in flakes upon the sides of the flues.  Perhaps the old remark that “smuts” or “blacks” falling to the ground on a sultry day are a sign of thunder is traceable to a similar action.

The most important practical result of the early experiments with frictional electricity was Benjamin Franklin’s great discovery of the identity of lightning and the electric spark.  One day in June, 1792, he went to the common at Philadelphia and flew a kite beneath a thundercloud, taking care to insulate his body from the cord.  After a shower had wetted the string and made it a conductor, he was able to draw sparks from it with a key and to charge a Leyden jar.  The man who had “robbed Jupiter of his thunderbolts” became celebrated throughout the world, and lightning rods or conductors for the protection of life and property were soon brought out.  These, in their simplest form, are tapes or stranded wires of iron or copper attached to the walls of the building.  The lower end of the conductor is soldered to a copper plate buried in the moist subsoil, or, if the ground is rather dry, in a pit containing coke.  Sometimes it is merely soldered to the water mains of the house.  The upper end rises above the highest chimney, turret, or spire of the edifice, and branches into points tipped with incorrosive metal, such as platinum.  It is usual to connect all the outside metal of the house, such as the gutters and finials to the rod by means of soldered joints, so as to form one continuous metallic network or artery for the discharge.