current of about 4 or 5 amperes. This in a test-room or laboratory is extremely useful, enabling very fine adjustments of resistance, and consequently of current or pressure, to be obtained when desired.

The reader is referred to the description of the same make of rheostat shown in Fig. 178.

Improved Rheostat.

The object of the rheostat, invented over forty years ago by Wheatstone, is to provide an electric resistance which can be varied continuously.

The instrument shown in Fig. 178 is an improved form (due to Lord Kelvin) of Wheatstone's rheostat, in which the wire is guided from one cylinder to the other by a fork carried along through the requisite range by a nut travelling on a long screwshaft. This screw-shaft carries a toothed wheel which turns the two cylinders by means of toothed wheels attached to their shafts. A watch-spring, as in Jolin's improvement of Wheatstone's rheostat, keeps the wire always tightened to the proper degree. A leather buffer at each end of the range of the nut acts as a guard against overwinding in either direction.

In a high resistance rheostat the conducting cylinder and the wire are both of platinoid, a metallic alloy having properties which make it specially suitable for the purpose. It has very high electric resistance, very small temperature variation of resistance, and its surface remains almost or altogether untarnished in the air. On account of the last-named property the contact between the wire and the conducting cylinder, and continuity in action, which was a great difficulty in the old form of apparatus, is very complete.

In a low resistance rheostat the conducting cylinder in this instrument is made of brass, nickel-plated so as to avoid tarnishing, and the wire used is copper, also nickel-plated. The rheostat can be supplied to carry currents as high as 30 amperes. The relation between material resistance and current for these rheostats is as follows, viz.—

Continuously Variable Rheostat.

Fig. 179 illustrates another slightly different form of Kelvin's improvement on the original Wheatstone rheostat. The actual construction is the same as that described in the two preceding, Figs. 177 and 178, except that the one here shown is intended for finer and lighter work, as seen from the smallness of the parts and gauge of wire employed. The second cylinder is just behind the one shown in the figure. Fairly fine wire is used, and the rheostat so wound may have a resistance of about 100 ohms as a maximum, capable of carrying one or two amperes. the "Wirt" form of resistance, described in Practical Electrical Testing by the author, it forms a most useful type of resistance for small work, such as calibrating low resistance

Like

voltmeters, enabling a series of different readings to be taken without actually altering the number of cells in the E.M.F. used.

The reader should refer to the description of the principle of this form of rheostat which is given relative to Fig. 178.

Fixed Standard Low Resistances.

There are many different forms of these depending to a great extent on the value of the resistance, and also on the particular make. Fig. 189 shows a set of five different forms of standard low resistances made by Messrs. Crompton and Co., and primarily intended for use with the potentiometers made by them also. The resistances can, of course, be used for any other purpose than this which requires a standard resistance of accurately known value, capable of carrying large currents without sheet or sensible heating or alteration.

The one shown standing on end at the top is of a slightly different form to the rest, being of the tubular water-cooled type.

These resistances consist of a sheet or strip of metal, or a coil of wire, each provided with four terminals, two for connection to the circuit and two for connection to the potential leads.

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The smaller resistances take the form of a coil or spiral fixed in a mahogany frame, and also of flat strips either bent or straight, the largest size-300 amperes and over are of sheet or water-cooled type.

They are constructed of manganin, an alloy which has been thoroughly tested, and which has the great advantage that within ordinary limits of accuracy (say one part in 1000) no temperature correction whatever is necessary: but for measurements with the potentiometer requiring an accuracy exceeding this, a curve, giving the temperature value for the whole range of current that the instrument is capable of carrying, is supplied with each resistance.

Such a curve, together with other forms of adjustable and fixed standards of low resistance, are given in the author's work entitled Practical Electrical Testing.

Stand Coil Rheostat.

A most convenient form of current rheostat of a portable nature, at all events one that can be moved comfortably about any testing-room, is illustrated in Fig. 181. It consists of an iron frame or stand, of as light a construction as possible, in order to be light, between the top and bottom of which are stretched bare wire spirals of either iron or some high-resistance alloy. These coils are spaced sufficiently far apart to prevent them easily touching should the rheostat receive a slight knock, and are all connected in series, their junctions being connected to a multiple way switch seen on the top. This latter consists of several studs or blocks arranged in a circular form and having their upper surfaces turned up in the lathe so as to be quite level. A suitable spring lever pivoted in the centre of the ring of blocks is capable of turning almost once round between two stops and of making contact with each block as it passes over it.

One of the two terminals of this stand coil rheostat (seen on the top) is connected to the lever centre, and the other to one end of the series of coils. Thus the resistance between the terminals can be varied from nothing (i. e. short circuit) to the maximum by as many steps as there are contact blocks on the switch. A great mistake, which is usually made by rheostat makers, is to use