ELECTRIC MACHINE, or Dynamo Static Machine (now extensively employed in therapeutics), for exciting electricity by fric tion or by static electric induction, as distin guished from dynamo electric machines in which electricity is excited by cutting magnetic lines of force. The excitation of electricity by friction was observed by the ancients, and the word electricity is derived from the Greek word electrum, signifying amber, in which the at tracting power of electricity after friction was first noticed. See ELECTRICITY, FRICTIONAL.
It was subsequently found that a glass rod or a rod of sealing wax also possessed this elec trical property when rubbed. Later on, other methods of exciting static electricity, such as by the electrophorus, were discovered. Such methods were, however, slow and laborious and better devices were sought for, resulting in the development of two general types of dynamo static machines, namely, frictional machines and induction machines, the first representing the rubbed glass rod or wax type; the second, the electrophorus type. These machines pro vided means whereby the rubbing and the in duction might be performed continuously and mechanically.
Frictional Electric The first continuous frictional machine, due to Nairne, consisted of a glass bottle, or hollow cylinder, mounted on a horizontal axis, well insulated, and turned by a winch or handle on suitable supports. (See Fig. 1). On one side of the cyl inder and pressing firmly against it, is placed the urubber,g a cushion of leather, c, stuffed with horse-hair, and sometimes faced with amalgam. From the upper edge of the cushion a silk flap, s, passes over the cylinder, reaching half way round. A brass cylinder, or a wooden cylinder plated with metal, P, termed the prime conductor, supported by a glass or ebonite rod, R, is placed as shown. A metallic comb is at tached to the left end of the prime conductor, as indicated in figure.
Before the machine is set in operation an amalgam of zinc and tin or other suitable metals, is sprinkled or pasted on the surface of the rubber. When the cylinder is turned elec
tric sparks are seen and heard to play on the cylinder from the edge of the flap, negative elec tricity being accumulated on the amalgam sur face and positive electricity on the surface of the cylinder, due to the friction between the glass and the amalgam coated surface of the rubber. As the cylinder is a non-conductor of electricity this positive charge is held on its sur face until the cylinder in revolving brings it be neath the comb, where it acts inductively on the prime conductor, P, attracting negative electricity to its near end and repelling positive electricity to its far end; the negative' electricity escaping across the points of the comb as an electrical breeze, or brush discharge, uniting with and neutralizing the positive electricity on the glass cylinder opposite the comb, which portion of the cylinder is then ready to take a positive charge as before when it again reaches the rub ber. When the prime conductor by these suc cessive charges of positive electricity has at tained a high potential, sparks several inches in length willjump from its far end to the hand or any other ground connection. To ob tain a continuous supply of electricity from the prime conductor, P, it is necessary to connect the rubber, c, to the ground, which is usually done by allowing a metal chain attached to a steel plate on the back of the cushion to lie on the floor or table, this permitting the negative electricity excited in the rubber to escape to earth. In this way also a negative stream of electricity may be drawn from the machine. The use of the amalgam on the rubber has the effect of largely increasing the conductivity of the cushion, and also provides a substance, a, be tween which and glass the surface friction for the exciting of static electricity is greater than that between leather and glass.