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Weston Standard Cell

The e.m.f. of an ordinary cell depends upon the materials employed and changes with the change in temperature and while supplying load, therefore, ordinary cells can not be taken as sources of constant e.m.f. which are required for electric measurements and either precision work.

  Weston Standard Cell

The Weston cell is such a cell whose e.m.f. remains constant for a longer period provided no appreciable current is drawn form the cell, therefore, such cells are never used as a source of energy but are used as a secondary standard of voltage for electrical measurements.

Its e.m.f. when constructed in accordance with the standard specifications is 1.01824, International volts or 1.01859 absolute volts are 20oC. This falls 0.04 mille-volts per degree centigrade use in temperature.

Weston standard cell is shown in. It consists of H shaped glass vessel, each leg being 25 mm in diameter containing concentrated solution of cadmium sulphate (CdSO4) in the upper part, then crystals of cadmium sulphate, paste of miraculous sulphate and cadmium sulphate over them mercury at the bottom of one leg and crystals of cadmium sulphate and cadmium amalgam at the bottom of second leg. The two limbs of the vassal are hermetically sealed. Cadmium sulphate solution acts as an electrolyte, mercury acts as a + ve electrode, cadmium amalgam (a solution of 1 part of cadmium in 7 parts of mercury) acts as a – ve electrode and paste of miraculous sulphate and cadmium sulphate acts as a depolarizer. Crystals of cadmiums sulphate are put in the cell in order to deep the electrolyte saturated. The connections of the cell into the external circuit are made by platinum wires sealed into the glass.

The following precautions are required to be observed while using Weston standard cells:

(i)    Since e.m.f. is only strictly constant on open circuit, therefore, care must be taken to ensure that no appreciable current is drawn form the standard cell while in use.

(ii)    Care should be taken also while moving the standard cell, as an appreciable shaking up of chemicals in the cell tends to cause variation in its e.m.f.

(iii)    The standard cell should be kept at dry places having uniform temperature of about 15oC to 20oC for the purpose of storage, so that hysterics effects due to change in temperature and possibility of leakage currents owing to  moistures on the intuiting materials between the terminals of the cell are  avoided.

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