RESISTANCE MEASUREMENT.] ELECTRICITY 45 inasmuch as it professes to represent iu electromagnetic measure a velocity of 10 9 centimetres per second, or, taking the original definition of a metre, an earth quadrant per second. It happens, by a curious accident, that the mercury unit and the ohm are very nearly equal, the latter being expressed in terms of the former (according to Pehms and Hermann Siemens; see Wieclemann, Bd. ii. 2, 1074) by the number 1 0493. One of the earliest instruments for furnishing a graduated resistance was the rheostat, brought into use by ^ heat- stone, but also invented independently by Jacobs at St Petersburg about 1840. It consisted of two cylinders of equal diameter, one of wood and one of brass. A wire, whose extremities were in connection with the metallic axes of the cylinders, was wound in opposite directions round the cylinders. The axes of the cylinders were connected with two binding screws by means of sliding contacts. The part of the wire which does not lie on the metal cylinder is the only part that produces resistance between the binding screws ; and, by winding and unwinding, we can increase or diminish the resistance continuously to a known extent, means being provided for measuring the angular rotation of the metal cylinder. We shall not slop to consider the defects of this instru ment, which is now never used for delicate work. Its place is taken by resistance boxes, containing coils of wire whose resistances are different multiples of the unit of resistance (in this country always the ohm). The reader will find a full account of the methods by which the standards are reproduced in the collected reports of the Committee on Electrical Standards. The usual material for the wire of resistance coils is German silver. Most of the copies of the ohm issued by the British Association were made of an alloy of two parts of silver to one of platinum. The great advantage of alloys is that the varia tion of resistance with temperature is small for them ; in the PtAg alloy, for instance, it is less than a tenth of the value for an average pure metal. To secure insulation the wires are carefully coated with silk, and after winding the coil is immersed in melted paraffin. To get rid of electro magnetic and inductive effects, the wire on resistance-coik is doubled on itself before being wound, so that, when a current passes through the coil, there are always two equal and opposite currents at each point. The terminals are formed by stout pieces of copper rod, whose resistance is either included in the coil, or is so small that it may be neglected. The connections for small resistances are managed by means of mercury cups, with pieces of amalgamated copper at the bottom, on which the copper electrodes are made to press. For ordinary purposes the coils are arranged in a box (fig. 23), the terminals l>eing stout pieces of brass fixed on the ebonite lid ; Fig. 23. conical brass plugs inserted between these pieces serve to throw the coils in and out of circuit. The box represented in fig. 23 is specially arranged for use in Wheatstone s bridge. In E, F,G wo have a series of coils, 1000, 100, 10, 10, 100, 1000 ; these are used for the arms of the bridge. In A,C,D there are sixteen coils, 1, 2, 2, 5,, 10, 20, 20, 50, &c., which give us any resistance of a whole number of ohms from 1 up to 10,000. In actual use the resistance to bo measured is inserted between A and G, D and E are connected by a stout piece of copper, the galvanometer is inserted between F and A, and the battery between E and G. The resistances of the anna of the bridge are taken equal, and as near the resistance to be measured as possible. In this way the resistance of any conductor may be very quickly found to an ohm. If it is desired to go farther, we may proceed thus. Suppose that we have found that a resistance lies between 5 and 6, put in the arm FE 100, and in FG 10, let the resistance in DCA, when there is a balance, be 57, then the resistance of the conductor is T Vs x 57, or 5 "7. Similarly we might go to a second place of decimals by putting 1000 in FE and 10 in FG. There is a limit, however, to this process, because the increase in the resistance of the arm decreases the sensibility of the bridge. Another method is to balance as nearly as possible, and then interpolate by taking the deflection of the galvanometer. Suppose, for instance, in the above case, that, with 5 ohms in DCA, the deflection was 21 in one direction, and, with 6 ohms, 9 in the other direction, then, taking the deflection proportional to the deviation from balance (see formula for 5 above), we have 21 resistance = 5 + -- . 1 = 57. 30 We might also construct small graduated resistances; and this would enable us to use smaller arms in the bridge, and thus increase the " sensibility " when used to measure small resistances. Owing to the multiplication of con nections, there is a limit to the ordinary resistance box arrangement, The difficulty may be evaded to a certain extent by using conductivity boxes, according to Sir W. Thomson s suggestion, where the resistances are arrangsd abreast, so that a small alteration of the resistance is brought about by adding on a very great resistance to the multiple arc. The rheostat principle has been used by Poggendorff in his rheocord for producing small resist ances. He stretches two platinum wires side by side; oil these is strung a hollow box filled with mercury, whose longitudinal motion is read off on a scale. If this arrange ment be thrown into any circuit by means of two binding screws connected with adjacent terminals of the wires, the parts of the two wires up to the bridge give a small resist ance, which may be adjusted at pleasure. In the quicksilver agometer of Miiller (Wiedemann, i. 1GO), the resistance is formed by a column of mercury of variable length. We may remark here that difficulties equally arise in constructing very large resistances. To get such within reasonable compass the wire must be ex ceedingly thin and the insulation very good. Messrs Warden and Muirhead have wound coils of fine wire, giving a resistance of 100,000, and have constructed in compact form resistance boxes up to 1,000,000, or a megohm, and beyond. They have also given practical form to a suggestion of Phillips to utilize the resistance of carbon, by drawing fine pencil lines on ebonite or glass ; they mix plumbago with the pulp in the ordinary process of paper manufacture, and thus produce a species of carbon paper. A strip of this about 21 in. long and 5 in. broad gives a resistance of about 50,000. This seems a valuable invention; but we are not aware how far it has stood the test of practical use. Selenium and tellurium have been proposed as material for high resistances, but owing to the variability of their resistance under the action of light, (fee., they are unfit for the purpose. The best method for comparing resistances with great Conduc tivity boxes, rheocord, &e. Kirch- hoff s gradu ated wire, & Fiji. 24. accuracy is the modification of Wheatetone s bridge intro
duced by Kirchhoff (fig. 24).