A laboratory manual of electrotherapeutics / by William James Herdman and Frank W. Nagler.

  • Herdman, William James.
Date:
[1898], ©1898
Catalogue details

Licence: Public Domain Mark

Credit: A laboratory manual of electrotherapeutics / by William James Herdman and Frank W. Nagler. 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.

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    thing which tends either to diminish the E. M. F. or to increase the resistance of a cell must cause a diminution of the current strength. The chief effects of polarization are the following,— i. The presence of hydrogen bubbles on the copper plate sets up an E. M. F. in an opposite direction to the E. M. F. acting from the zinc to the copper plate. The effective E. M. F. is then equal to the difference between the two. 2. The effective surface of the copper plate is de- creased by the highly resistant hydrogen gas, and then the internal resistance of the cell is increased and the current is correspondingly decreased. Methods of Preventing Polarization.—i. Any mechanical method of removing the hydrogen from the negative plate such as brushing the plate or keeping the liquid of the cell agitated would diminish polarization. This method has never proved to be very satisfactory and is now obsolete. 2. Polarization may be prevented by surrounding the negative plate of the cell by some oxidizing material which is capable of forming a chemical combination with the nascent hydrogen. In this manner the two chief causes of polarization are prevented. Nitric acid (HNO ), chromic acid (CrCy, manganese dioxide (Mn OJ and Cal- cium hypochlorite (Ca (C10)2) are commonly employed. 3. By placing the negative plate in some salt whose metallic element can easily be replaced by nascent hydro- gen, polarization can be entirely prevented. The chem- istry of the Daniell and Chloride of Silver cells will illustrate this principle.