A system of electrotherapeutics as taught by the International correspondence schools, Scranton, Pa.

  • International Correspondence Schools
Date:
1899-1902
Catalogue details

Licence: Public Domain Mark

Credit: A system of electrotherapeutics as taught by the International correspondence schools, Scranton, Pa. Source: Wellcome Collection.

Provider: This material has been provided by the Harvey Cushing/John Hay Whitney Medical Library at Yale University, through the Medical Heritage Library. The original may be consulted at the Harvey Cushing/John Hay Whitney Medical Library at Yale University.

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    CONDUCTION, CONVECTION, AXD RADIATION 11 • The increase in temperature of a conductor does not, as a rule, keep step with the number of watts spent in heating it, because heat, like electricity, i« subject to losses from various causes. These losses may take place in three different forms, through conduction, convection, and radiation. In conduction, the heat is transmitted from particle to particle in the body itself. In a heated electric conductor the heat would leak from the interior to the surface. The transmission of heat by convection takes place in liquids and gases only, as it requires a certain mobility of the molecules of the substance. If a beaker of water is placed over a Bunsen burner, the bottom will become heated and the particles of water situated immediately above the same will be heated by conduction. When heated they become lighter and rise to the surface, thereby giving room to other particles. In this manner a constant stream of water will carry off the heat supplied by the bottom of the vessel. When radiation goes on there is no direct contact between the source of the heat and the recipient of the same. The ether is in this case the transmitting medium and is able to transfer the heat from one place to another without itself being heated. Fur instance, the sun radiates its heat through space and in this manner supplies the earth with its necessary heat, but without heating the intervening medium. ] 2. Limit to Increase of Temperature.—A heated conductor may give up its heat by either one or all of these forms. If exposed to the air, the latter will become heated and, in passing away, set up a current that will constantly carry off heat from the conductor. The amount of surface a conductor exposes to the air will therefore be of importance in determin- ing the rise in temperature. Thin wires heat more rapidly than thick wires of the same material, partly because they have less surface to dispose of the heat and partly because of the higher resistance. Suppose, for instance, that two wires, one ^ and the other jSj inch in diameter, have to carry the same current. The