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ELECTRICAL MEASURING ' INSTRUMENTS 135

Make a knife-contact by flattening a piece of heavy cop-
per Wire as shown in Figure 117. Solder a piece of flexible
wire, such as “ lamp cord,” at the ot- er end. It is well to
fit the contact with a small wooden handle, made by" boring
out a piece of dowel.

The instrument is now practically complete.

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

 

WIRE

    

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CONTACT

FIG. I r 7. — Knife-Contact.

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 “ elec-
trical 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-cov-
ered wire from an electrical dealer and cut into the fol-
lowing lengths, laying it straight on the floor but using
care not to pull or stretch it.

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