through the sole plate of system A. These screws can be raised or lowered at pleasure, and by means of a gauge the plates of system B can be adjusted to exactly midway between, and parallel to, the plates of system A. The complete Leyden stands upon three vulcanite feet attached to the lower side of the sole plate of system A.

In order that the instrument may not be injured in carriage, an arrangement, described as follows, is provided, by which system B can be lifted from off the three glass columns and firmly clamped to the top and bottom plates of system A.

The bolts fixing the corners of the plates of system B are made long enough to pass through wide conical holes cut in the top and bottom plates of system A, and the nuts at the top end of the bolts are also conical in form, while conical nuts are also fixed to their lower ends below the base plate of system A. Thumbscrew nuts, f, are placed upon the upper ends of the bolts after they pass through the holes in the top plate of system A.

When the instrument is set up ready for use, these thumbscrews are turned up against fixed stops, g, so as to be well clear of the top plate of system A; but when the instrument is packed for carriage, they are screwed down against the plate until the conical nuts

=

A standard.stacle censer genera, new,

and Fig. 224 a man of the top with the connections of the five conE and C are the two double terminals of the

densers aside the ox.

enter, and from the diagrammatic figure it will be noticed that anipulation of the plugs to insert capacity is just the reverse to of an ordinary resistance box, in that you plug in the front set of

holes to increase the terminal capacity, since capacities in parallel sum up like resistances in series.

It will also be seen that a plug inserted in a back hole short-circuits that particular condenser, and any of the others that may be in, providing a plug is also in the front hole of the same condenser.

Hughes' Magnetic Balance.

A form of Hughes' magnetic balance, which is useful for rapidly determining the relative magnetic qualities of two or more samples of material, is shown in Fig. 225. It consists of a long bench or table supported on two standards on a base fitted with levelling screws. At one end (the right-hand) of this raised bench is a magnetizing solenoid,

in which is inserted the specimen to be tested, and at the other a small balancing solenoid to neutralize the inductive effect of the magnetizing coil by itself on a magnetometer needle suspended between the two coils. A permanent steel compensating magnet (seen to the extreme left) can be turned round a pivot passing through the centre of a vertical scale fixed to the base, and its deflections made to neutralize the various stages of magnetization of the specimen itself. The method is a "zero" one, and the indications or position of the magnetometer needle is read by forming an image of a vertical pin, fixed just in front of the needle box, in the moving plane mirror attached to the needle, which is half covered by a fixed plane mirror immediately in front of it. Thus the zero position of the needle is

when the two images coincide. Professor S. P. Thompson has shown that, when the distance from needle to centre of compensating magnet is 2-3 times the latter's length, the angle through which the magnet is turned up to 60 is proportional to the magnetic force due to the iron core or specimen in the magnetizing coil.

Hughes' Induction Balance.

Fig. 226 shows a Hughes' induction balance, together with the interrupter, telephone, and other accessories. It consists of two pairs of flat circular coils supported as shown, one pair at each end of a baseboard. The bottom coil of each pair is permanently fixed on the

top of a short central pedestal. These are in series, and constitute the primary coils of the balance, being connected to two terminals marked P on the baseboard. The other coil of each pair, which is supported close over its respective primary, is a flat circular coil of similar size, but having a hole through its centre large enough to receive a wooden cup as shown. These latter coils, constituting the secondary of the balance, go to terminals S. The left-hand one is permanently fixed on the two pillars seen in the figure, while the righthand one is supported on three pillars, the two on the extreme right

forming a kind of axis or fulcrum, about which the coil can be raised or lowered by a set screw working on the third pillar being pulled downwards by an elastic band seen on the hook. A current interrupter, consisting of a spring strip pressing against, and making electrical contact with, the teeth of a toothed wheel, which is driven by clockwork, is seen with its case off between the two pairs of coils, and is for the purpose of "making and breaking" the primary circuit in which it is inserted. A Bell's telephone is seen on the right of it, and a long wedge-shaped metal strip of zinc fixed to a millimeter scale on the left. This can slide between the adjust

able guides seen on the left-hand top coil. Circular discs of various metals are provided for placing in the right-hand cup. It should be understood that the four coils are as nearly alike in every detail as it is possible to make them, both as regards size, form, wire, number of turns, etc.

The Permeameter.

A useful piece of apparatus, known as the "permeameter," is illustrated in Fig. 227, by means of which the magnetic quality of different materials can easily and rapidly be found. The method is essentially a workshop one, and the principle of it is due to Professor S. P. Thompson.

FIG. 227.

The arrangement consists of a somewhat massive hollow rectangular-shaped block of good soft wrought iron forged to the shape shown. Inside this is a magnetizing solenoid wound on a thin brass tube with thin flanges or ends, its length being just that between the insides of the block ends. The sufficiently long rod of magnetic material to be tested, having its lower end faced quite true, passes freely but closely through the top end of the block, down through the solenoid, and beds on the carefully "faced" inside of the bottom end of the block. The protruding end of the rod has a metal pin through it, which is caught by a double hook on the lower end of an ordinary spring balance, the top end of which is suspended by a gut cord passing over a fixed pulley and attached to the lever shown. This permeameter is also fitted with an arrangement for testing the