Induction Machines.—In frictional machines there arc two ways in which energy is expended—in friction, and in drawing away the two excited and attracting surfaces. Much of the force expended in friction results in heat, and only a fraction (sometimes a small one) in electricity. Of that spent in drawing away which is the less consider able, the whole results in increased potential. Machines are therefore very desirable where, with a small initial charge, a constant supply of electricity may be got by the latter method. The electrophorus (q.v.) is a machine of this kind, and has been known since 1776, and Nicholson's doubler, another, since 1788. But the action of these has only been on a small scale. Lately, however, induction machines of great power have been made, the powers of which quite eclipse the older frictional machines. The best known of these is Holtz's machine (invented 1865), which will suffice to show the general construction of such.
Holtz's machine consists of two glass disks, A and D (fig. 2), of very thin glass fully covered over with shellac varnish. The one, D, is somewhat larger than the other, A. The plate D is stationary, and is kept in its 'place by four cular grooved rings of vulcanite V, V, V, V, placed in horizontal glass rods, which themselves are supported by upright glass pillars a, a, a, a. By this method of support the plate D may be turned round to rest in any position. The upright lass pillars rest on the sole of the instrument, and pass through two solid blocks of vulcanite L. L, which serve to knit the whole frame well together. In L two brass rods R, R.% are fixed, which, on the end next the revolving plate A, each support a horizontal row of teeth facing the plate A; and on the other, two sliding rods with vulcanite handles II, IF, which can be adjusted to distance, and which form the poles of the machine. The revolving plate A is fixed in a canite spindle, the ends of which move in the blocks L, L'. This plate is made to revolve at great speed by a handle and multiplying belts. It is unbroken, and revolves as close to the plate I) as to keep clear of it. The stationary plate D, which is shown in fig. 3, has two holes cut in it like those at P and P'. The lower edge of the one opening, and the upper edge of the other, lie along the line of the teeth of the two poles already mentioned. On the side of the plate D, away from the revolving plate, are stuck two coolings or armatures of varnished paper C, C', and from those protrude two tongues, also of varnished paper, slightly turned into the openings of the fixed plate, and towards the revolving plate. The size of the revolving plate is shown
in fig. 3 by a dotted circle.
Suppose now we have to work the machine, and let us take one whose plate (a usual size) is 2 ft. in diameter. We first of all put the two poles B and B' in contact—we see that the teeth lie opposite the line TT', and that the revolving plate revolves in the opposite direction to the tongues P, .P', or which is the same thing, that the revolving plate passes an opening before coming to the row of teeth. We next take a sheet of vulcanite about the breadth of an armature, and rub it with cat's skin (making it —), and hold it close to the arma ture C, and then make the plate revolve. We at once know if the machine is working by a rushing sound.
We now gradually withdraw the two balls at the poles, and a rush of straight, bright but not very dense sparks leap across between the two for the first 2 or so. When the distance becomes greater than that, brushes proceed from each end, and there is a fine purple glow in the central space. If we with draw the poles to 5 or 6 in., two well-defined brushes are seen, one at each pole, the larger and most fully formed being the brush at B'. The — brush at B' is much smaller. If now the hand be placed on one pole and the other hand presented to the other, sparks of 2 in. are got which produce a most painful stinging sensation on the hand, but cause no twitching at the joints of the arm. When the poles are at a dis tance of half an inch, paper and other combustible substances may be kindled by the spark. On examination it is found that C, or the armature first touched by the excited vulcanite, is — (as shown in fig. 3), and that the other is --F; that B, which is opposite C, is —, and B'+. If the machine be viewed in the dark, long --F. brushes are seen at P and the tooth opposite it, and — stars at P' and at the tooth opposite or attached to B'. The plate is+ below—that is, after passing the negative armature; and — above, after passing the +armature. If the motion of the plate be reversed, the electricity of the poles changes sign or ceases altogether, when the machine must be excited anew. If kept moving in the same direction, and allowed to rest only for short intervals, it may be kept in action for hours together without renewed excitement.