Electro-medical instruments and their management : and illustrated price list of electro-medical apparatus / by Schall & Son.

  • Schall & Son.
Date:
1911
Catalogue details

Licence: In copyright

Credit: Electro-medical instruments and their management : and illustrated price list of electro-medical apparatus / by Schall & Son. Source: Wellcome Collection.

    16/252 (page 4)
    hydrogen on the positive, metal. The quantity of the produced gas is in proportion to the strength of current. In a cell consisting, for instance, of zinc, sal-ammoniac and silver, the silver becomes covered with gas bubbles very shortly after the circuit is closed, and then the cell consists only of zinc and hydrogen, which has a very much lower E.M.F. than zinc and silver. The strength of current decreases in consequence of this formation of gas, and the first consideration in constructing a cell is to prevent this action, which is called polarisation. Depolarisation.—It can be achieved in different ways : either by shaking off the bubbles mechanically, or by chemical action. The positive metal is then surrounded with materials containing plenty of oxygen, which unites eagerly with the hydrogen and becomes water, annihilating thus the gas bubbles. For this reason some cells contain nitric acid or chromic acid, the Leclanche cell manganese di-oxide, etc. The depolarisation, as we call this process, works perfectly in the chloride of silver or Daniell cell. Such cells are therefore called constant elements, compared with a chromic acid cell, in which the depolarisation is less perfect, and which is called inconstant, because its strength of current decreases after a short time. A Leclanche cell is constant if it is worked for short intervals only or with weak currents ; but it is inconstant if it has to yield a current too strong in proportion to its size, or if it remains closed for many hours without rest. Units.—It became necessary soon to introduce units, in order to be able to express in figures the amount of E.M.F., or the strength of current and the amount of resistance, etc. An International Congress of Electricians agreed to derive the electrical units from the measures for length, weight and time (centimetre, gramme, and second), in order to be able to compare the effects produced by electricity with those produced by other physical forces, and moreover they agreed to name the different units for E.M.F., strength of current, resistance, capacity, etc., after the physicists, who have by their great discoveries, materially developed the knowledge of electricity, such as Volta, Ampere, Ohm, Faraday, etc. The unit of the E.M.F., or potential, which has been chosen is very near the E.M.F. of a Daniell cell, and has been called Volt. The unit of resistance is Ohm. It equals the resistance of a mercuric column of i square millimetre sectional area, and i*o6 metre length at a temperature of 320 F. The unit of strength of current is called Ampere. It is the current which an E.M.F. of 1 volt produces in a circuit, the resistance of which is 1 ohm. 1 ampere is too much for medical purposes, and therefore its one-thousandth part, or 1 milliampere, has been adopted as unit for measures of intensity. A source of electricity with an E.M.F. of 1 volt passing through a circuit, the resistance of which amounts to 1,000 ohms, produces in it a current of 1 milliampere. Watt is the expression for the product of volt and ampere ; for instance, a 16 candle-power incandescent lamp requiring either 100 volts and 0 5