plates without touching them. They deposited their electricity

FIGURE 5.-The same machine, viewed from above.

through which it escaped under a high pressure, became the

generator of static electricity. The boiler was insulated by glass legs, and became negatively electrified. The jets of steam conveyed the positive electricity and deposited it upon a metal plate studded with points, upon which the jets were directed. This machine proved very powerful, but difficult to manage, and totally unfit for general use. It made a deafening noise, and saturated everything near it. One of these machines gave a spark of twenty-two inches.

To Holtz, of Berlin, we do FIGURE 6.—An apparatus for gener- not owe the discovery of the erating frictional electricity by first induction - machine, as steam, devised by W. Armstrong.

The legs upon which the boiler many suppose. His apparatus rests are of glass. The nega- was not perfected until 1865. tive electricity generated by the Although the original model machine when in action accu- seems crude in comparison positive electricity is collected with our present instruments, by the comb upon which the still it cannot be denied that steam-jets are directed. it contained the principle

mulates upon the boiler, and the

FIGURE 7.-One of the oldest models of a cylindrical static machine. The rubbers are grounded, and a Leyden-jar is connected with the positive electricity stored in the receiver.

which formed the starting-point of all the later improvements;

and many of the mechanical details of the original instrument are to-day generally used.

There is a modified form of an induction-machine which is now sold quite extensively to the medical profession. It is known as the Toepler-machine, or the Voss-machine. It can be made with one or more revolving plates. The fixed plates are larger than the revolving ones, and have usually a central opening. They may, however, be divided or perforated centrally. They are furnished with paper collectors and discs of

*

FIGURE 8.-The original model of Holtz's induction-machine with vertical plates. The same inventor also perfected a machine without windows or armatures, in which two horizontal plates revolved in opposite directions. This machine is shown in a subsequent cut. tin-foil. The revolving plates have metal buttons attached to one of their faces. These buttons impinge upon metal brushes as the plate is revolved. The buttons rest on tin-foil cemented to the glass. The fixed plates are placed as close as possible to the revolving plates.

It is claimed that this machine will work in all weathers. I have not found this to be strictly fact; although it is not as *I have lately had one so divided, which works admirably.

much affected by dampness as an ordinary plate machine.* This machine is usually not encased-a defect which I have remedied, with satisfactory results. It is lighter and less expensive than the improved patterns of the Holtz-model; but it is far less satisfactory in medicine, because it generates a much smaller quantity of electricity and has less intensity. The spark elicited may be a moderately long one (when compared with the radius of the revolving plates); but it is rather a thin spark at best-thus confirming the view expressed by me respecting the "quantity" generated.

THE PRINCIPLES OF STATIC INDUCTION.

The application of the principles of static induction as demonstrated in the machine devised by Holtz, is difficult to fully explain without devoting more time to the general subject of electrical induction than is deemed wise. It may be roughly summarized, however, as follows:

Any body when electrified has the power, to a greater or less extent, of exerting (even through an intervening substance, which in this instance consists of a plate of glass,) a peculiar effect upon the electrical state of another body closely adjacent to it in position. It tends to draw from the opposed body that variety of electricity which it does not itself possess. Now, if an intervening substance happens to exist between the two bodies, the electricity drawn toward it by induction may be deposited upon the corresponding side of that substance; and a proportionate amount of electricity of the opposite variety is abstracted from the intervening body. Hence, the intervening body becomes either positively or negatively electrified on one side, as the case may be.

In the induction machine, the intervening substance happens to be the revolving glass plate; and the opposed bodies are the two paper collectors and the two metal combs of the machine, which are separated by the revolving plate of glass.

In all induction-machines, the charge is practically constant when once established—provided the mechanism be perfect and the plates kept absolutely dry. Under such conditions, it ought never to fail to produce its full effects when the wheels are set in revolution. This is a great desideratum in medicine. model only one stationary and one Both were circular in shape. The

In the original Holtz revolving plate were used.

*This opinion is supported, moreover, by the fact that some manufacturers of these machines give to their purchasers explicit directions respecting the drying of the plates.

stationary plate had openings or "windows" cut in it. Paper collectors were glued to the stationary plate; so made as to project from it and to come in close contact with, and to face the openings in the stationary glass plate. The revolving plate was insulated by legs of glass, while the stationary plate was not. Metal combs were used as terminal attachments to the inner end of the two poles of the machine. They faced the revolving plate and almost touched it.

FIGURE 9.-The stationary plate of the original Holtz, showing its armatures and windows, with the projections upon the armatures.

You will find all of these mechanical features practically preserved in the improved models of to-day. The revolving and stationary plates have been increased in number; simply to augment the quantity of electricity generated. The stationary plates are no longer circular; they are made in two pieces, to allow of "windows." Two paper-collectors are glued to each stationary plate. These terminate in points which project into the "windows" made by dividing the plates. The poles of the machine have metal combs on one end, and a brass ball at the other. Extra combs have been added to draw off residual electricity which accumulates in excess; but these are "grounded."

Furthermore, the machine has been encased, simply to protect it from atmospheric changes. Cat-skin rubbers have been. added. They are of use only as a means of exciting the plates when, from any cause, induction shall have ceased. We call them the "chargers" of the machine. (See Figure 11).

There have been many mechanical modifications made, from