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.

    71/608 (page 51)
    which is a common form of electric supply, a consideration of the self-induction of the circuits is absolutely necessary if serious errors are to be avoided. Conductors supplied with alternating current, especially when they are wound in the form of coils so as to set up strong mag- netic fields, are found to have an apparent resistance which is very much greater than they have when tested by steady currents, and the introduction of an iron core into a coil of wire which is carrying an alternating current can easily be shown to produce a still further increase in its apparent resistance. From this it is clear that Ohm's law (§ 24), which specifies that the magnitude of a current in a circuit is determined by two factors only—viz., the electromotive force and the resistance—needs to be modified for the case of varying currents by the introduction into the calculation of a new factor : namely, the self-induction of the circuit. It is the opposing electromotive force, or hack E.M.F. of self-induction, which increases the apparent resistance. Sometimes it is convenient to use the expression ohmic resistance to signify the resistance of a circuit as measured for steady currents, and to distinguish it from the virtual resist- ance of the same circuit when varying currents are being considered. The word impedance is generally used for virtual resistance. It is very necessary for medical men to have some knowledge of the phenomena of electromagnetic induction, and of the behaviour of circuits carrying varying currents. Without an acquaintance with this part of the subject it is impossible to understand the induction coil, or the construction and manage- ment of the various forms of medical apparatus in which alter- nate currents are used. High-frequency phenomena, too, can only be comprehended by those who have given attention to the effects of self-induction and mutual induction. The pre- ceding paragraphs may serve to indicate the importance of the matter, but for a proper elucidation of the laws of varying currents and of electromagnetic effects in general the reader must consult a modern textbook of electricity. 48. The Induction Coil.—One of the most interesting of the early observations in connection with electromagnetic induction was that shocks and bright sparks could be produced from a single voltaic cell if the circuit contained spiral coils of wire. From the last paragraph we learn that these effects depend upon the self-