In order to use the Wheatstone bridge, it is necessary to have a set of resistances of known value. The resistance of any unknown circuit or piece of apparatus is found by comparing it with one of the known coils. It is just like going to a store and buying a pound of sugar. The grocer weighs out the sugar by balancing it on the scales with an iron weight of known value, and taking it for granted that the weight is correct, we would say that we have one, five, or ten pounds of sugar, as the case may be.
The Wheatstone bridge might be called a pair of "electrical scales" for weighing resistance by comparing an unknown coil with one which we know has a certain value.
The next step is to make up some standard resistance coils. Secure some No. 32 B. & S. gauge single-cotton-covered wire from an electrical dealer and cut into the following lengths, laying it straight on the floor but using care not to pull or stretch it.
1/2 ohm coil—3 feet 1/2 inch
1 ohm coil—6 feet 1 1/4 inches
2 ohm coil—12 feet 2 1/2 inches
5 ohm coil—30 feet 6 1/4 inches
10 ohm coil—61 feet
20 ohm coil—122 feet
30 ohm coil—183 feet