A text-book of electro-therapeutics and electro-surgery.

  • Butler, John (Physician)
Date:
1880
Catalogue details

Licence: Public Domain Mark

Credit: A text-book of electro-therapeutics and electro-surgery. Source: Wellcome Collection.

Provider: This material has been provided by the Harvey Cushing/John Hay Whitney Medical Library at Yale University, through the Medical Heritage Library. The original may be consulted at the Harvey Cushing/John Hay Whitney Medical Library at Yale University.

    294/336 (page 288)
    the intensity of current we use a number of cells, the positive element of one cell being united to the negative of the next, and so on, leaving a positive at one end of tlie series, and a negative at the other, free to form the ultimate poles. When the human body, or any portion of it, is included in the circuit, it is generally essential that a number of cells be used ; as the resistance of any tissue in the body is very great, indeed, several million times greater than any of the metals. Some tissues of the body, however, conduct much better than others; when, therefore, we wish to pass a current through a part which is a good conductor, and when the circuit is short, we do not use so many cells to produce the same effect as when the opposite condition exists. For example: Suppose we wish to pass a current through a part, the resistance of which is nine hundred ohms and we use 20 Darnell's cells for the purpose, the internal resistance of each cell being 5 ohms, we get the effect of one-fiftieth of a veber of actual current. The equation reads: — . r 20 E _ 20 J_ 20 R + r ~~ 100 + 900 0r 50 part of a veber of actual current is transmitted during a second of time. If the resistance be 4000 ohms, (which it often is) the equa- tion with the same battery would read: , p 20 E 20 1 p . A- G = 20R+7 = 100 + 4006 01' 205 ot tl veber' From this it will be readily seen that, knowing the number of cells and the electro-motive force, gives us no idea of the amount of current flowing, unless we also know the resistance of the circuit. Hence, when the poles are placed close together on the body, and especially when the part operated upon is a good conductor, 20 Daniell's cells represent a strong current, whereas when the contrary is true, the same number of cells transmit only a very mild current. The differences of conductivity of the tissues are about as follows, the highest numbers representing the highest resist- ances :