Fig. 16. Circuit for Wheatstone Bridge Measurement
To test the cell, open both knife switches and place switch S on the left hand point. Place the cell in the clips with the terminal connected to the gold leaf in the clip connected to binding post A. This makes the gold leaf the anode of the cell. Now close the battery switch applying about 6 volts to the bridge. Balance the bridge by adding or removing resistance in the rheostat arm until the galvonometer gives no deflection. The resistance of the cell in the dark is read direct from the markings on the open switches on the rheostat box.
Now close the single pole knife switch lighting the cell and again balance the bridge. The latter reading will be the light resistance of the cell, gold anode.
For convenience in recording the data a sheet of paper should be ruled into columns headed, date, voltage applied, resistance dark, resistance lighted (gold anode), resistance dark, resistance lighted (copper anode) and remarks. The cells should be numbered and the corresponding data sheet headed with the same number so that changes in the cell can readily be detected and improvements noted. A separate sheet should be provided for each cell.
Now increase the voltage to 9 volts and take another set of readings with the cell both dark and lighted. The values obtained are recorded and the voltage increased further. The cell is tested with gradually increasing voltages till the maximum voltage possible without heating the cell has been applied. Heating of the cell can usually be noted by a faint crackling noise being given but it is advisable to feel the surface of the mica occasionally to detect any heating.
It may be found on examining the data obtained that the resistance of the cell has varied with different voltages. We can classify the cells by calling a cell in which the resistance increases with an increasing voltage the A type, those in which the resistance falls off with an increase of voltage the B type and those in which changes of voltage cause little or no change of resistance the C type.
Now reverse the cell in the clips making the copper plate the anode and repeat the above series of tests and record the values in the proper columns on the ruled sheet. It will be found in many cases that the reversal of the current thru the cell has increased the resistance. A cell showing this characteristic is polarized while one in which the reversal of the current does not alter the resistance to any great extent is non-polarized.
The cell is now to be treated with alternating current for a period of five minutes by opening the battery switch, cutting the galvanometer out of the circuit and closing the double pole switch. Current will flow thru the ratio arms and the cell. Care should be taken that the current is not heavy enough to heat the cell excessively. If this occurs reduce the current by adding resistance till heating is not detected.
The cell is again tested according to the directions already given both for its light and dark resistances with the current flowing in both directions thru the cell. If the difference between the dark and light resistance has increased it is safe to say that the cell will be sensitive to light and the treatment should be continued to develop this property. The quickest method of doing this is to select from the various readings the voltage that gives the greatest difference in the dark and light resistance. Use this voltage to make one test after each treatment with A. C. till the sensitiveness of the cell to light has reached a fair value. When the resistance in the dark is ten times as great as in the light the cell is suitable for experimental working and treatment can be discontinued.
Should the cell have practically no resistance when first put in the testing set it is useless to go thru the entire series of tests till the resistance has been raised. Treat the cell with A. C. repeatedly till the resistance is brought up to a fair amount, at least 500 ohms, then make the tests outlined previously.