WHEATSTONE. 465 WHEEL. circuits." From this instrumpiit, which had five neeJlos, grew the system of electric telegraphs wbicli was extensively used in Great Britain. Alter the five-needle telegraph came apparatus where but two needles were employed, then the letter-showing dial telegraph, the type-printing telegraph, the magneto-electric dial telegraph, and the automatic telegraph system, in which great speeil of transmission was obtained. In a paper on binocular vision before the Royal Society in 1S3S, he explained the i)rinciple of the stereoscope (q.v.), an instrument of his inven- tion subsequently improved by Brewster. The polar clock (q.v.), another invention of Wheat- stone, was an interesting application of Brews- ter's discovery that the plane of polarization of the light of the sky always made an angle of 90° with the sun. In 1840 he showed that by means of electro-magnetic regulating devices a num- ber of clocks far apart coulil be kept going synchronously from a central clock. Wheat- stone's bridge (q.v.), though not the invention of this famous electrician, but of Hunter Christy, was first brought into general use through his efl'orts in 1843, and has proved a fundamental method in the measurement of electrical resist- ance. Wheatstone was one of the first to recog- nize the importance of Ohm's law and to apply it in electrical measurements. He also had a share in the development of the dynamo, de- vising a method for making the magneto-ma- chines then in use self-exeiting by employing a shunt circuit passing around the field magnets. Wheatstone was far more suecessfnl as an ex- perimenter and inventor than as a professor, and, though he held his chair for a number of years, he rarely lectured in public, owing to his great diffidence. In fact, many of his important discoveries were communicated to the Royal So- ciety by Faraday. This characteristic did not prevent his contributing many interesting papers to scientific journals and the Proceedings of various learned societies, and these were col- lected and jiublished in 1870 by the Physical Society of London under the title of Scientific Papers of Sir Charles Wheatstone. WHEATSTONE'S BRIDGE. An important and much used method for the measurement of electrical resistance. Employing a battery, a galvanometer, and auxiliary resistances, it is possible to determine the resistance of a con- ductor with a high degree of accuracy. The principle of the method will be readily under- stood by reference to the accompanying diagram. Let B be a battery whose terminals are con- nected with the ends of a straight wire of German being a resistance whose value is known and the latter the material whose resistance is to be measured. The _two branch circuits unite at 6 and return to the battery. At t, the junction of K and X, one terminal of the galvanometer. (J, is attached, while the other, S, is connected with a slider which can be moved along the wire so as to make contact at any desired point. Let the current at o be at a higher potential than at 6, then according to Ohm's law there will be a fall of potential along the wire depending upon the resistance, and, since ah is a uniform conductor on the length, this will be proportional to the length. The same fall of potential as along the wire «6 must also take place through R and X, consequently some point on ab will be found which is at the same potential as t. Where there is a dili'erence of potential, there must result a fiow of electricity which will deflect the needle of the galvanometer. We can determine when the two points are at the same potential by mov- ing the sliding contact until there is no deflec- tion of the needle on closing the circuit. Con- sequently there must be the same fall of poten- tial from a to t and from f to fi as from a to S and from S to 6. It has been shown that the fall of potential along oS and S6 is proportional to the distances of these parts from the ends of the wire, which we can denote by A and B, therefore the resistance R must be to the- resistance X as A is to B. In practice R is selected so that the measurement will be made when the point of contact is at the centre of the wire rather than at the ends, for, as can be readily seen, the degree of accuracy is far greater when A and B are of nearly the same value than when one is large with respect to the other. The wire bridge, as the apparatus is termed, is useful for many purposes and answers well in explaining the principle, but the apparatus is generally constructed with a series of resistance coils in the form known as the Post-Office box (so called from the fact that the operation of the telegraph lines in England is under the control of the Post-Office) . In this arrangement there is a series of fixed ratios formed by coils of wire taking the place of the stretched wire used above and other coils so arranged as to give R any value between 1 and 10.000 ohms. The ratio coils generally have their resistances 10, 100, and 1000 ohms, which permits a range of measurements from 1 . , cm silver, f/7;. which is stretched along a scale. The current tlowing from-<j to b divides at a. a part of it traversing the bridge wire, as ab is termed, and a part flowing through R and X, the former to 1.000.000 ohms. See Resi.st. ce. Elec'trYcal. Consult : Kempe, Electrical Testing (6th ed., London, 1001) ; Tliompson, Elementary Lessons in Magnetism and Electricity (rev. ed,. New York, 1901). WHE'DON, Daniel Deni-Soij (1808-8.5). A Methodist Episcopal minister. He was born at Onondaga. N. Y. ; graduated from Hamilton Col- lege in 1828, and studied law. He was professor in Cazenovia Seminary. 1830:'tutor in Hamilton College. 1831-.32; professor of languages in Wes- leyan University, Middletown. Conn.. 1833-43; professor of rhetoric in Michigan L'niversity, 1845-54: editor of the Methodist Pericn-, Xew York, 1856-84. He wrote a Commentary on the Neir Testament (1860-80) ; The Freedom of the Vill as the Basis of Human Responsibility (1804) ; Essays. Revieirs, and Discourses (1887). WHEEL (AS. htceol. h>riol. hweoicol, hweolil, Icel. hjol, E. Frisian wel, wheel; connected with
