in like manner be disturbed, a primary current being induced on first introducing the magnet, and a secondary one on withdrawing it. It is obvious that if by any contrivance contact with the battery could in the first example be rapidly made and broken; or, in the second, the magnet be as quickly immersed and withdrawn, we should procure a rapid series of currents moving alternately in opposite directions; and on this is founded the construction of all the magneto-electric and electro-magnetic machines.

Numerous forms of electro-magnetic machines have been suggested for medical purposes; and it is really not a matter of any importance which you employ, provided care be taken to have the one you have chosen so arranged as to allow of a sufficiently copious development of electricity. As we have seen that in all such contrivances a small voltaic current furnishes the initial force, it is important to have this completely under command, and to be able to make and break contact with the inducing apparatus, with the utmost facility and rapidity. You may break contact with the battery, if you please, by means of a ratchet or cog-wheel; but this is often inconvenient, as it renders the services of an assistant necessary. On this account, an automatic apparatus is always to be preferred. I believe I proposed the first of these several years ago, in the Annals of Philosophy; but this, as well as all others I have seen,

is much inferior to one constructed by an ingenious philosophical instrument-maker, Mr. Neeves, of Broad Street, Holborn, and this is the only one I ever now employ. It possesses the advantage of simplicity, facility of employment, quantity and intensity of the induced electricity, together with the additional recommendation of low price.

This consists of a wooden bobbin, with a hollow axis. About thirty feet of thick insulated copper wire are wound on it, and over this about a thousand feet of very fine insulated copper wire, the ends of which are soldered to a couple of binding-screws fixed in the base of the instrument. The former is the coil in which the initial or inducing current is intended to circulate; the latter is the secondary coil, whose electricity is to be disturbed and thrown into motion, to form the

* Fig. 22. A, the wooden bobbin, on which is wound the double coil of wires. B, C, the screws connected with the ends of the fine coil, with conductors affixed. D, the apparatus for breaking battery contact. E, single pair of plates (Smee's arrangement) connected with the screws, F, G. H, H, the conductors by which the induced currents are directed to any object.

induced current.

One of the ends of the primary

inner coil of thick wire is connected with the zinc plate of a simple battery; the other end of the wire surrounds a small horse-shoe of soft iron, and is then soldered to the lower end of a bent rod of brass, whose upper end carries a small screw furnished with a platinum point, which presses on a plate of the same metal fixed to a transverse bar of thin brass, having at the end suspended over the poles of the horse-shoe, a disk of soft iron. When the fixed end of this bar is connected with the copper or silver plate of the little battery, the disk of iron is rapidly attracted by the ends of the horse-shoe, which acquire a powerful magnetic force. In an instant, the contact between the platinum wire and plate being broken, the current is arrested, and, the horseshoe losing its magnetism, the elasticity of the brass bar causes it to fly up and again bring the platinum point and plate in contact, when the same series of alternate attractions and repulsions occur. In this way you see the brass bar rapidly vibrate, and produce a loud humming musical sound, varying in pitch according to the amount and amplitude of the vibrations; and contemporaneously a rapidly succeeding series of induced currents traverse the coil of fine wire. If I now grasp in my hands a pair of brass cylinders connected with the ends of the fine coil, a series of currents of high intensity, and rapidly succeed

ing each other, rush through the arms, producing a most painful and nearly intolerable sensation. You observe that a bundle of iron wires is placed in the hollow axis of the bobbin. The use of this is obvious enough; for these wires becoming a series of powerful temporary magnets add their inducing power to that of the initial current, and greatly increase the tension of the excited electricity. Indeed, by withdrawing the bundle of iron wire, you may diminish most materially the severity of the shocks produced by this instrument, and thus enable you very conveniently to adjust their force according to the case under

treatment.

Sometimes the coils of wire are placed vertically, and then the exposed ends of the bundle of iron wire in the centre is made to attract the little Fig. 23.

disk of iron, and thus break contact. This form of apparatus is preferred by some.

If you reflect for a moment on the principles on which the construction of these very convenient arrangements is founded, you will at once see that you cannot obtain, by the aid of either of these machines, a series of positive and negative currents in a definite direction; that neither of the conducting-wires is capable of being regarded as negative and positive. This you can readily understand from the results of the experiment I showed you just now with the galvanometer. Each of the conducting-wires of this instrument conveys, alternately, currents in opposite directions. The wires, at the rate the bar is now vibrating, convey about 500 currents per minute, each being alternately negative and positive. To demonstrate the truth of this statement I have here on a glass plate a piece of paper moistened with a mixed solution of starch and iodide of potassium. I place on it the platinum extremities of the conducting-wires of the electro-magnetic apparatus; the currents pass, electrolytic action occurs, the iodine is severed from the potassium, and being set free, stains the starched paper. On examining the paper you will find the purple stain of iodide of amidine at both points where the platinum wires touched the surface. Now, as the iodine is invariably liberated at the place where positive electricity enters the body containing it, we have