plug in the measuring arm does not tend to tighten or loosen all the rest of the plugs; moreover, there are fewer plugs to manipulate, and each plug is occupied. The resistance coils themselves are generally wound on brass or copper bobbins, with silk-covered manganin wire, which should first be aged by heating for about ten hours to a temperature of 140° C, to remove the slight tendency to change in resistivity which would otherwise present itself.
For the accurate comparison of resistance coils it is usual to make use of the Matthiessen and Hockin bridge, and to employ the method of differential comparison due to G. Carey Foster.[1] On a board is stretched a uniform metallic wire a b, generally of platinum silver. The ends of this wire are connected to copper blocks, which themselves are connected to a series of four resistance coils, A, B, and P, Q (fig. 4). A and B are the coils to be compared, P and Q are two other coils of convenient value.
Fig. 4.
Over the stretched wire moves a contact maker S, which makes contact with it at any desired point, the position of which can be ascertained by means of an underlying scale. A battery C of two or three cells is connected to the extremities of the slide wire, and the sensitive galvanometer G is connected in between the contact-maker and the junction between the coils P and Q. The observer begins by moving the slider until the galvanometer shows no current. The position of the coils A and B is then interchanged, and a fresh balance in position on the bridge is obtained. It is then easily shown that the difference between the resistance of the coils A and B is equal to the resistance of the length of the slide wire intercepted between the two places at which the balance was found in the two observations.
Let the balance be supposed to be attained, and let be the position of the slider on the wire, so that and are the two sections of the slide wire, then the relation between the resistance is
Next, let the position of A and B be interchanged, and the slide-wire reading be ; then
Hence it follows that , or the difference of the resistances of the coils A and B is equal to the resistance of that length of the slide wire between the two points where balance is obtained.
Various plans have been suggested for effecting the rapid interchange of the two coils A and B; one of the most convenient was designed by J. A. Fleming in 1880, and has been since used by the British Association Committee on Electrical Units for making comparison between standard coils with great accuracy (see Phil. Mag., 1880, and Proc. Phys. Soc., 1879).
Fig. 5.
In all very exact resistance measurements the chief difficulty, however, is not to determine the resistance of a coil, but to determine the temperature of the coil at the time when the resistance measurement is made. The difficulty is caused by the fact that the coil is heated by the current used to measure its resistance, which thus alters in value. In accurate comparisons, therefore, it is necessary that the coils to be compared should be immersed in melting ice, and that sufficient time should be allowed to elapse between the measurements for the heat generated in the coil to be removed.
The standard resistance coil employed as a means of comparison by which to regulate and check other coils consists of a wire, generally of manganin or platinum silver, insulated with silk and wound on a brass cylinder (fig. 5). This is soldered to two thick terminal rods of copper, and the coil is enclosed in a water-tight brass cylinder so that it can be placed in water, or preferably in paraffin oil, and brought to any required temperature. In the form of standard coil recommended by the Berlin Reichsanstalt the coil is immersed in an insulating oil which is kept stirred by means of a small electric motor during the time of making the measurement. The temperature of the oil can best be ascertained by means of a platinum resistance thermometer.
For the measurement of low resistances a modification of the Wheatstone's bridge devised by Lord Kelvin is employed. The Kelvin bridge consists of nine conductors joining six points, and in one practical form is known as a Kelvin and Varley slide. Modifications of the ordinary Wheatstone's bridge for very accurate measurements have been devised by H. L. Callendar and by Callendar and E. H. Griffiths (see G. M. Clark, the Electrician, 38, p. 747). A useful bridge method for measurement of low resistances has been given by R. T. Housman (the Electrician, 40, p. 300, 1897). These and numerous modifications of the Wheatstone's bridge will be found described in J. A. Fleming's Handbook for the Electrical Laboratory and Testing-Room, vol. I. (1903).
References:— F. E. Smith, "On Methods of High Precision for the Comparison of Resistances," Appendix to the Report of the British Association Committee on Electrical Standards, British Association Report (York, 1906), or the Electrician, 57, p. 976 (1906); C. V. Drysdale, "Resistance Coils and Comparisons," British Association Report (Leicester, 1907), or the Electrician, 57, p. 955 (1907), and 60, p. 20 (1907); J. A. Fleming, "A Form of Resistance Balance for Comparing Standard Coils," Phil. Mag. (February, 1880); "A Design for a Standard of Electrical Resistance," Phil. Mag. (January 1889); G. Aspinall Parr, Electrical Measuring Instruments (1903); W. H. Price, The Practical Measurement of Resistance; A. Gray, Absolute Measurements in Electricity and Magnetism (1900); Rollo Appleyard, "The Conductometer," Proc. Phys. Soc. London, 19, p. 29 (1903); also Proc. Inst. Civ. Eng. 154 (1903); and Proc. Phys. Soc., London, 17, p. 685 (1901). (J. A. F.)
WHEEL (O. Eng. hwēol, hweohl, &c., cognate with Icel. hjōl, Dan. hiul, &c.; the Indo-European root is seen in Sanskrit chakra, Gr. κύκλος, circle, whence "cycle"), a circular frame or solid disk revolving on an axis, of which the function is to transmit or to modify motion. For the mechanical attributes and power of the wheel and for the modification of the lever, known as the "wheel and axis," and of the mechanical powers, see Mechanics. The most familiar type of the wheel is of course that used in every type of vehicle, but it forms an essential part of nearly every kind of mechanism or machinery. Vehicular wheels in the earliest times were circular disks either cut out of solid pieces of wood, or formed of separate planks of wood fastened together and then cut into a circular shape. Such may be still seen in use among primitive peoples to-day, especially where the tracks, if any exist, are of the roughest description, and travelling is heavy. The ordinary wheel consists of the nave (O. Eng. nafu, cf. Ger. Nabe, allied with "navel"), the central portion or hub, through which the axle passes, the spokes, the radial bars inserted in the nave and reaching to the peripheral rim, the felloe or felly (O. Eng. felge, Ger. Felge, properly that which
fitted together, Teut. felhan, to fit together). From the monuments we see that the ancient Egyptian and Assyrian chariots had usually six spokes; the Greek and Roman wheels from four to eight. (See further Carriage and Chariot; also Tire; and articles on Bicycle; Tricycle; and Motor Vehicles.)
WHEEL, BREAKING ON THE, a form of torture and execution formerly in use, especially in France and Germany. It is said to have been first used in the latter country, where the victim was placed on a cart-wheel and his limbs stretched out along the spokes. The wheel was made to slowly revolve, and the man's bones broken with blows of an iron bar. Sometimes it was mercifully ordered that the executioner should strike the criminal on chest and stomach, blows known as coups de grâce, which at once ended the torture, and in France he was usually strangled after the second or third blow. A wheel was not always used
- ↑ "On a Modified Form of Wheatstone's Bridge, and Methods of Measuring Small Resistances," by Professor G. Carey Foster. Proc. Soc. Tel. Eng. (1872), 1.
