15. Stöhrer's transportable induction apparatus, both the smaller and the greater, consist of a mahogany box which is divided into two parts by means of a partition. In the left side is a movable zinc-carbon element, already described, and in the other the induction apparatus. By means of the clamps which secure the zinc and coal to the partition, the battery current is conducted to the primary spiral and to the interrupter. This consists of a quadrangular iron rod fastened to an easily-moving spring, which may be attracted and set free from an electro-magnet composed of a bundle of iron wires, and allows of the motion of a hammer and the consequent induction of the secondary spiral. This latter can be lifted vertically by means of a graduated rod. A brass spring, having a screw, leans against the hammer, and serves to produce, through its greater or less action, a change in the strength and times of the induction shocks. Four screw-stands have the office of conducting the current to the body; those marked P give the primary current, and those marked S the secondary current. Modifications of the strength of the current can be produced: a. For both currents, by varying the arrangement of the battery-glasses and the action of the hammerspring. b. We can further check the primary current by connecting the screw-stands of the secondary current by means of the attached wire bow, and by lifting the spiral by its rod. c. If in using the secondary current the wire bow is removed, the strength of the current is increased when the spiral is lifted. Various attachments are found in the drawer on the right side of the apparatus.

The larger apparatus differs from the smaller in thata. It has two battery-glasses; b. Its hammer is better constructed, consisting of an iron beam whose recoil is regulated by a spiral spring; c. As the primary current, even by complete lifting of the induction spiral, is not sufficiently weakened for certain cases, Stöhrer has lately added a copper tube in the interior of the induction apparatus, which, lifted

by a small graduated rod, slides over the primary spiral, and, in connection with the elevated closed induction spiral, reduces the primary current to a minimum.

16. Du Bois-Reymond's apparatus is most conveniently

set in operation, when an intense current of no great duration is wanted, by means of a small Grove's element; when, on the other hand, a continuing strong current is needed, it is best to connect it with one or two Bunsen's elements, according to its size which are filled after Poggendorf's direction (see page 14), or with the chamber-battery of Stöhrer, described on page 104. In the latter case, it is also advisable to use the greater apparatus, with 5-inches-long magnetizing and induction rolls, whose primary spiral consists of about 500 windings of a strong wire of 1.2 mm., and whose secondary of about 10,000 windings of wire of 0.25 mm.; while, when using Grove's element, the smaller apparatus, with 24-inches-long magnetizing and induction rolls, whose primary spiral consists of about 250 windings of a strong wire of 1.2 mm., and whose secondary of about 500 windings of 0.25 mm., is fully sufficient.

The wire going from the zinc pole of the element is secured in a standard on the front end of the apparatus, which at the same time is the starting-point of the magnetizing

spiral. From A the wire passes to the horse-shoe B, around which it forms a spiral, and then goes to a horizontal wooden cylinder, C, filled with a movable bundle of iron wires, around which it also winds in a spiral form, and ends in the brass upright D. This latter is secured to a brass piece, E, which at its front end is pierced for the reception of the steel screw F. The conducting wire going from the carbon (or platinum) connects with a perpendicular column, G, which receives, above, the steel spring of an iron hammer, H, that, as soon as the apparatus is set in motion, strikes uninterruptedly on the steel screw For on the horse-shoe B, and thus continues or breaks the connection between the wires. The positive current goes then from the carbon of the element to the brass column G, from this to the iron hammer П, as far as the point of contact with the steel screw F, thence through the brass piece E to the wire of the wooden cylinder C; it then passes to the horse-shoe B, and ends in the standard A, which receives the conducting wire coming from the zinc. Besides the wooden cylinder filled with iron rods already mentioned, the apparatus has a second, I, the socalled "sled," which, by means of a track on the pedestal, can be moved backward and forward over the small wooden cylinder, and may cover this more or less fully. The greater cylinder is, as has been mentioned, surrounded by a twenty times greater number of windings of a wire which has about one-fifth of the thickness of that of the primary wire; its beginning and end are taken up by standards placed on the posterior part of the apparatus, which serve also for receiving the conducting wires that carry the current of the second order to the body to be electrized. The extra-current is conducted from D, and from a standard near by, which is by means of a wire in connection with A.

The extra-current has its greatest intensity when the bundle of iron wires is shoved fully into the cylinder, the sled removed, and the action of the magnetizing spiral withdrawn; the more the wire bundle is pulled out and the sled

pushed in, the weaker is the current. The current of the second order has, on the contrary, its greatest intensity when the sled fully covers both wooden cylinder and iron rods; the more the rods are drawn out, the sled removed, and the action of the magnetizing spiral diminished, the weaker is this current. The number of intermissions may be lessened or increased by the greater or less approximation of the steel screw F to a platinum plate placed on the middle of the hammer H.

Often the Du Bois apparatus is furnished with a measuring-rod, graduated to inches and lines, which is secured to the sled, so that comparative measurements of the contractility of different muscles at one time, or of the same muscle at different times, may be made. Yet this arrangement is mostly superfluous for the first purpose, and for the second insufficient, when the apparatus is not at the same time furnished, as is Duchenne's, with a compass, which measures the strength of the galvanic element, and, if necessary, regulates it. But even then the influence of the outer temperature of the dry and perspiring skin-not taking into account the variation which the apparatus shows through the deposition of coal on the platinum plate, through the greater or less approximation of the steel screw to the hammer, etc.-is so considerable, that the making of such measurements is not worth the loss of time required.'

We have, in the previous pages, described a number of magneto-electrical and volta-electrical induction apparatuses,

1 [In the first edition of this work I gave it as my opinion that the induction batteries of Kidder were superior to all others made in this country. Since then, the instruments made by the Galvano-Faradic Manufacturing Company have been introduced, and are preferable in every respect to any heretofore constructed. They are fully described in the Appendix. Kidder's apparatus is, however, good of its kind. The galvanism is derived from either one or two Smee's cells, according to the size of the apparatus. Thebox contains the batteries, and also a bottle in which the dilute sulphuric acid employed may be kept when the apparatus is not in use.

a consideration of which will give a tolerably complete idea of their gradual improvement. As to the first, we noticed those

Fig. 13 represents the galvanic cell, and Fig. 14 the whole arrangement. A full description of the apparatus is given in the published pamphlet of the inventor. Dr. Kidder also makes a very portable apparatus.

It is set in action by a Grenet's cell, a very convenient form of battery composed of a zinc and two carbon plates, which dip into a mixture of dilute sulphuric acid and bichromate of potash. The elements are contained in a glass