The value of electrical treatment / by Julius Althaus.

  • Althaus, Julius, 1833-1900.
Date:
1895
Catalogue details

Licence: Public Domain Mark

Credit: The value of electrical treatment / by Julius Althaus. Source: Wellcome Collection.

Provider: This material has been provided by the Francis A. Countway Library of Medicine, through the Medical Heritage Library. The original may be consulted at the Francis A. Countway Library of Medicine, Harvard Medical School.

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    Ohms and Volts. Ohm's law forms the base of all our knowledge of galvanic electricity. It is to the effect that the current- strength is directly proportional to the electro-motive force, and inversely proportional to the resistance encountered in the circuit of the battery; in other words, the current- strength is equal to the electro-motive force divided by the E resistance. This may be expressed by the formula C=t7. A unit of resistance was first proposed by Werner Siemens in 1849, and defined as the resistance of a column of mercury one m^tre long, and one square millimetre wide. The British Association afterwards proposed their absolute unit (B.A.U.), which was called ohm, and adopted by the International Congress of Electricians, which met in Paris in 1881. The Congress, however, modified the unit in such a way that at present an ohm represents a column of mercury 106 centimetres long and I square millimetre wide. An ohm is therefore equivalent to I06 Siemens unit. This is called a legal ohm, in order to distinguish it from the real ohm, which has not yet been measured with absolute accuracy, but is about o3 per cent, less than the legal ohm. The units introduced by the Paris Congress are called absolute, a term first used by Gauss, who in connection with Wilhelm Weber reduced the magnetic and electric forces to units of length, mass, and time, without reference to arbitrary units. These original units were expressed in millimetres, milligrammes and seconds; but, at Paris, it was decided, on the proposal of Lord Kelvin, that the base of the units should be centimetre, gramme and second; and this is usually designated by the letters C.G.S. The Congress termed the unit of electro-motive force a volt; and this is of nearly the same value as the current given by a single cell of Daniell's battery. The exact value of that cell