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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    The last of these only will be used in finding the relation between the current passing through a conductor and the heat developed in it. This formula W = C2 X R shows that when heat is produced in an electric conductor, it is not proportional simply to the number of amperes passing, but to the square of the same. If, for instance, 2 amperes is flowing and transformed into heat, 4 amperes will not simply double the quantity of heat developed, but will increase it 4 times. Why this increase should be pro- portional to the square of the current-strength may not seem quite clear, and a practical example may help to make the matter better understood. When chemical energy is trans- formed into electrical energy, as, for instance, in a voltaic cell, the amount of material consumed per hour is proportional to the strength of the current. Suppose six Daniell cells are con- nected in series and are sending a current of 1 ampere through an external resistance. As the consumption of zinc per cell amounts to .043 ounce per ampere for each hour, the total amount of zinc consumed amounts to 6 X .043 = .258 ounce. If it is desirable to increase the current-strength to 2 amperes, while the resistance remains constant, then the E. M. F. must be doubled and therefore the number of cells increased to 12. As now each cell is transmitting a current of 2 amperes, twice the amount of zinc must be consumed, or .086 ounce per cell, and the number of cells now being 12, the total amount of zinc consumed is 12 X .086 = 1.032 ounces. This is 4 times .258 ounce, the original amount. We see then that to double the current-strength, 4 times as much chemical or electrical energy had to be developed, the electrical energy being proportional to the amount of material chemically transformed. 10. Conclusions.—From these considerations we come to the conclusion that the greater the number of watts transformed into heat the more rapidly the temperature must rise. Con- trarily, the higher the specific heat of a conductor, or its capacity for heat, the more slowly will this rise take place, and, finally, the greater the mass or weight of the conductor the more time will be required to effect a rise in temperature.