Manual of gynecology / by D. Berry Hart and A.H. Freeland Barbour.

  • Hart, D. Berry (David Berry), 1851-1920.
Date:
1905
Catalogue details

Licence: Public Domain Mark

Credit: Manual of gynecology / by D. Berry Hart and A.H. Freeland Barbour. 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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    The Erg or unit of work, the work done in overcoming unit force through unit distance, i.e., in moving a mass through a distance of one centimetre against the force of a dyne ; and Unit Strength of Magnetic Pole.—The unit magnetic pole is of such a strength that when placed at a distance of 1 cm. in air from a similar pole of equal strength it repels it with a force of one dyne. We are now in a position to understand the definition of the units referred to in the explanation of Apostoli's method. As that method deals with Current Electricity in which the positive and negative poles are in properties the same as magnetic ones, these units are called Electro-magnetic. Electro-magnetic Absolute Units.—(1) Unit Strength of Current is that of a current such that if one centimetre length of its circuit be bent into an arc of one centimetre radius it will exert a force of one dyne on a unit magnetic pole placed at the centre of the circle of which the arc is a part, so as to be always a centimetre away from the current. (2) Unit Quantity of Electricity, that quantity of electricity which is conveyed by current of unit strength in one second. (3) Unit of Difference of Potential or of Electro-motive Force, exists between two points when it requires the expenditure of one unit of work (Erg) to bring a unit of + electricity from one point to the other against the electric force. (4) Unit of Resistance is possessed by a conductor when unit difference of potential between its ends causes a current of one unit of quantity per second to flow through it. The first two of these absolute units were found to be inconveniently small and the last two inconveniently large, accordingly a committee of the British Association devised a system of practical units in which they substitute for the fundamental units centimetre and gramme, the Earth's quadrant (1,000,000,000 centimetres) and too,000*000.000 °f a gramme. Electro-magnetic Practical Units.—(1) The Volt1 is the practical unit of Electro-motive force and is 100,000,000 absolute units. (2) The Ohm1 is the practical unit of Resistance, and is 1,000,000,000 absolute units. (3) The Ampere1 the practical unit of Strength of Current, is that furnished by a Volt through an Ohm and is ^0 °f the absolute unit. In medical electricity, however, the strength of the current is measured in milliamperes. (4) The Coulomb1 is the practical unit of Quantity of current elec- tricity, and is ^ of the absolute unit. 1 These four terras commemorate tlie names of four famous electricians: Aiessandro Volla, who shares with Galvani the discovery of current electricity; G. S. Ohm, whose law regulating the strength of current electricity has been given above ; Andre Ampere, the founder of the .science of electro-dynamics; and Charles A. de Coulomb, the inventor of the tort-ion balance and demon- strator of the law that electrical attraction and repulsion vary inversely as the square of the distance.