Medical electricity : a practical handbook for students and practitioners.

  • Jones, H. Lewis (Henry Lewis), 1857-1915.
Date:
1913
Catalogue details

Licence: In copyright

Credit: Medical electricity : a practical handbook for students and practitioners. Source: Wellcome Collection.

Provider: This material has been provided by Royal College of Physicians, London. The original may be consulted at Royal College of Physicians, London.

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    when the wire is introduced, produces this transfer, is called the electromotive force from the one body to the other along the path marked out by the wire. If the potentials at different points of a conductor are different, there will be an electric current from the places of high to the places of low potential. The idea of electrical potential may be illustrated by comparing it with pressure in the theory of fluids and temperature in the theory of heat. If two vessels containing fluids are put into communication by means of a pipe, fluid will flow from the vessel in which the pressure is greater into that in which it is less till the pressure is equalized. Again, if two bodies at different temperatures are placed in thermal communication, either by actual contact or by radiation, heat will be transferred from the body at the higher temperature to the body at the lower tempera- ture till the temperature of the two bodies becomes equalized. Similarly, when two electrified bodies are put into electric com- munication by means of a wire, electrification will be trans- ferred from the body of higher potential to the body of lower potential. 9. Electrometers.—The only thing that can be observed in connection with electricity at rest is a difference of potential. It is possible to measure the quantity of electricity driven through certain instruments just in the way that a quantity of water driven through a water-meter can be measured, and some of these instruments will be discussed in a later paragraph ; but for the present we can only appreciate electrical charge by observing a difference of potential, and electroscopes and electro- meters are instruments for showing or measuring differences of potential. The gold-leaf electroscope has been shortly referred to above. The divergence of the leaves of this instrument may be taken as an indication that the knob or disc, or way by which electricity enters the instrument, is at a different potential to its surround- ings ; but obviously it does not give us more than the roughest indication of the amount of difference of potential. In cases where there is a great difference of potential a delicate gold-leaf electroscope is likely to be spoilt by the violent repulsion of the leaves, so rougher forms may he used—e.g., leaves of thin alu- minium or even pith balls suspended by linen threads may be used instead of the more delicate gold-leaf. If it is required to measure a difference of potential, an electro-