Sometimes even prolonged treatment will fail to raise the resistance, in this case test the cell for polarization by reversing in the clips. If strongly polarized the cell may be a good generator, i.e., will give a current under the influence of light. To test this, connect the galvonometer to posts A and D of the set, put battery switch on left hand point and light the cell by closing the single pole knife switch. If the galvonometer gives a deflection it indicates that current is being generated in the cell. This cell should be reserved for use as a generator, this property being increased by short circuiting the terminals and exposing the cell to light periodically until it will generate a fair amount of current. The current generated by these cells is a true photo-electric current, no chemical action taking place, the light rays being converted directly into electricity by some unknown action of the cell. The current flows from the copper plate thru the external circuit back to the gold foil terminal.
Should the cell after prolonged treatment with A.C., fail to increase in resistance or having increased in resistance and remain insensitive to light and not prove to be a current generator the cell must be classed as useless. It is seldom indeed that this occurs for a cell that shows even a slight change in resistance when lighted should be treated with A.C. from time to time and used in experiments and will eventually increase in sensitiveness sufficiently to be of value. When a cell proves intractable, the gold foil may be carefully removed by means of a stiff brush and the cell put back in the hot press to be reconstructed. Retreating with heat and pressure will often give a very sensitive cell.
Even when the cells are being used for experimental work the maturing process will be going on. The point of maturity and maximum sensitiveness of any cell has been reached when the dark resistance of the cell remains constant over a period of time. The maturing seems to affect the cell by increasing the dark resistance, the light resistance remaining practically the same thruout. From this it will be seen that a cell with a very low resistance when first made may under proper treatment become extremely sensitive to light.
As previously mentioned the testing set may be used for measuring the resistance of the cells by the substitution method. The apparatus is connected as shown in [Fig. 17] when so used, a diagrammatic circuit being given in the insert. When the battery switch is on the left hand point the cell is in series with the battery and galvonometer. When the switch is moved to the right hand point the rheostat is in series with the battery and galvonometer. The method of measuring is to first determine the galvonometer deflection with the cell in the circuit and then by switching the rheostat into the circuit and adjusting it so the galvonometer gives the same deflection enabling the resistance of the cell to be read directly. The same series of tests should be gone through with this method as with the bridge circuit.
Fig. 17. Circuit for Substitution Method of Measurement
The substitution method has the advantage of indicating roughly the sensitiveness of the cell to light. With the cell darkened and the current flowing thru it the galvonometer will give a certain deflection, on lighting the cell the deflection will increase indicating a lowering of the resistance. The determination of cell sensitiveness is more rapid and this circuit can be employed when the cell has a low resistance at first and it is necessary to increase it before tests can be made. With the substitution method of measurement it is advisable to use a rather insensitive galvonometer or meter since the current at times may reach a value that would damage a delicate instrument. A tangent galvonometer with heavy windings will be found most suitable.
To determine if a cell is sensitive enough to close a relay the substitution circuit may be used with a relay connected in place of the galvonometer. If the cell is suitable for the purpose the relay will close when the cell is lighted.