WHEATSTONE
2074
WHEEL
to 3,667,494,000 bushels, that of the United States being over 695,443,000 bushels. In the same^year the seven states showing the largest yield were Minnesota (94,080,000); Kansas (62,068,000); South Dakota (46,-720,000); Indiana (40,981,000); Nebraska (39,5i5,ooo); North Dakota (36,105,000); and Illinois (31,500,000).
Wheat'stone, Sir Charles, an English inventor and scholar, was born at Gloucester in February, 1802. He began making musical instruments, and made many experiments in sound, inventing an instrument that was played by a piano in another room and so seemed to j)lay itself. His first paper was New Experiments in Sound. He^extended his studies to light and electricity, invented a number of instruments, and with Cooke took out a patent for an electric telegraph in 1837. He invented the stereoscope, the cryptograph and a telegraphic barometer and thermometer, by which an observer at the foot of a mountain could read the instruments at the top. Scarcely a year passed without a paper from him on some difficult scientific subject or without an invention. He was made professor in King's College, London, a fellow of the Royal ^Society, a member of all the leading foreign scientific associations, and, after the completion of his automatic telegraph, was knighted. He died at Paris, while perfecting an instrument for submarine cables, Oct. 19, 1875.
Wheatstone's Bridge, a piece of electrical apparatus devised by Sir Charles Wheatstone for measuring the electrical resistance of a conductor. It consists of a network of six wires arranged as shown in the accompanying diagram. Four resistances : R, Rlt Rs and X, are so joined in series as to form a closed circuit. In this circuit there will always be four junctions, namely, A, D, B and F. The next step in manufacturing a Wheatstone's bridge is to join a battery across two diagonally opposite junctions, say A and B, as shown in the
DIAGRAM OP WHEATSTONE S BRIDGE
diagram. The current entering at A divides into two parts, one flowing along the branches X and Rj, the other flowing along the branches R and Rg.
Next join the remaining two junctions, F arid Dt oy a circuit containing a galvanom-
eter. In general, the potential at F and the potential at D will be different; but by diminishing the resistance in R and increasing the resistance in R2, or vice versa, we can easily make the potentials at F and D the same. When this adjustment is reached, there will be no current flowing through the galvanometer G, even if the key K3 be closed. Let us now call the fall in potential between A and F (and hence also between A and D) Ex; and the fall in potential between F and B we shall denote by Eg. Then, by Ohm's law, we may write
Dividing one of these equations by the other, we have
YRi
=(&>•
Accordingly, if we know the value of one resistance R and the ratio of the other two, Rt and R2, we can at once obtain the value of the unknown resistance X. This is the purpose of the Wheatstone bridge. Consult any good textbook of physics for a discussion of this subject,
Wheel, a piece of wood or metal, usually round and either solid or open, fastened to a center piece, around which it turns. The center-piece is called an axle when the wheel turns on it, and an arbor or shaft when the wheel turns with it. Wheels are used to produce motion and are of different kinds, as wagon-wheels, water-wheels, car-wheels, steamboat-wheels, fly-wheels, .balance-wheels, crown-wheels, spur-wheels, band- or belt-wheels, ratchet-wheels, cogwheels and pinions, the kinds used in machinery. Carriage and wagon wheels are made of wood with a band of iron; car-wheels and machinery-wheels of iron, though sometimes of wood or brass; and clock and watch wheels of brass also. Carriage and wagon wheels are made with a central hub of wood, which turns on an axle, from which spokes radiate to the wooden rim of the wheel. An iron tire or hoop, heated hot, so that it expands, is fastened on the rim, and as it cools it tightens, connecting the parts very firmly. Car-wheels are made either of iron or steel, or iron and steel, or of iron, steel and wood combined, and usually are solid, with the edge or rim fitted to the rail. Water-wheels (q. v.) are used for turning machinery by water. The overshot wheel has boxes or buckets on the rim. The buckets on one side fill with water and their weight makes the wheel turn, the buckets emptying themselves as they reach the bottom. The undershot wheel has ilat boards, called floats, in^ stead of buckets, and is used where there is plenty of water. The turbine-wheel is
