A small two point battery switch and four double spring binding posts are also mounted on the base and wired as shown by the dotted lines. A fairly sensitive galvonometer is employed to indicate when the bridge is balance. A telephone receiver might be used for the purpose, opening and closing the circuit to cause clicking in the receiver, the bridge being balance when the noise is reduced to a minimum. The small center zero ammeters with the shunt removed make excellent galvonometers.
Ratio arms and a rheostat box having a maximum resistance of 100,000 ohms complete the apparatus necessary to make the tests on the cells. A laboratory set with this range is rather expensive and since our measurements need not be extremely accurate a good resistance box may be made from resistance units as used for motor starting and signal work. These units consist of an iron tube covered with asbestos on which is wrapped the resistance wire, the wire in turn being covered with a vitreous insulating material baked in place. These units can be purchased quite reasonable from any large electrical supply house in any resistance up to 150,000 ohms. For our purpose 15 units will be required, five, 200 ohm; five, 2000 ohm and five, 20,000 ohm; all tapped at the center, for the rheostat arm. For the ratio arms a single 1000 ohm unit tapped at the center is used. The rheostat arm units should be mounted in a box and heavy leads run to 12 single pole switches mounted on the top of the box. The method of wiring the resistances is shown in [Fig. 15], the switches being marked so the resistance in the circuit can be quickly determined. With this arrangement any resistance from 100 to 111,000 ohms can be obtained in steps of 100 ohms by opening the proper switches.
Fig. 15. Connections for Interior of Rheostat Box
The ratio arms are made by bringing leads from the ends and center of the 1000 ohm unit. Ratios other than one to one are not advised, for the cells are so sensitive to external influences that one system of measurement must be adopted and adhered to if the cells are to be compared. With the one to one ratio half the current flows thru the cells when the bridge is balanced.
The current for testing the cells can be obtained from a dozen three cell flashlight batteries. The battery should be connected to a multipoint switch so that any number of cells can be switched into the current as desired.
To prepare the various instruments for testing the cells with the bridge circuit connect as shown in [Fig. 16]. The diagrammatic wiring is shown in the insert in the illustration. The positive or carbon of the battery is connected to post A. The source of alternating current whether from the lightning mains or an induction coil is connected to the double pole switch, current for the lamp in the enclosed box being supplied to it thru the single pole switch.
