1851 Great Exhibition: Official Catalogue: Class X.: J. N. Hearder
439. HEARDER, J. N., 34 George Street, Plymouth — Inventor and Manufacturer.
Cast-iron compound horse-shoe permanent magnet, applicable to purposes requiring high magnetic power. The application of cast iron is new. This magnet consists of 24 plates, 2 inches wide, 13,3 of an inch thick, cast in the form of a horse-shoe, which is 161 inches long from the poles to the outside of the bend; the poles are 11inches as under, and the inside of the bend 31 inches wide. The 24 plates weigh about 72lbs., and are fastened together with three bolts and nuts. The poles are capped with cast iron, which concentrates the magnetic power in an extraordinary manner. The construction is very simple: the bars are cast from No. 1 pig iron as hard as green sand can make them, and they require no preparation to adopt them for magnetization. The soft iron caps render the grinding of the poles unnecessary. The attractive power of the magnet is scarcely inferior to that of a steel magnet of the same dimensions, whilst the economy in construction is nearly as 4 to 1; the cast-iron magnet weighing 72 lbs., lifts 140 lbs.
Powerful horse-shoe steel magnet, of 100 plates, adapted for purposes requiring high magnetic power. It weighs about 39 lbs., and will support nearly 250 lbs. with a round-faced keeper.
The Rev. Dr. Scoresby appears to have been the first to employ thin sheet steel for the construction of powerful permanent magnets. His idea being that, as magnetism appeared to reside principally on the surface of the metal, by multiplying the number of surfaces, the power of the compound bar would be increased. Mr. Hearder, many years since, constructed powerful magnets from cast-iron, and was certainly the first to use that material for the purpose; and the power which can be induced is certainly very great with the soft iron caps.—R. H.]
Medico-galvanic apparatus, with graduated regulator, employed to administer galvanic electricity. Its improved construction consists in the proper adjustment of the length and thickness of the generating or primary coil to the electro-motive force of the battery, by which the battery-surface is much reduced, and a higher amount of magnetism produced in the iron core.