Calibration of a Bridge-wire

The method gives us also the best means of calibrating a bridge-wire. Make an observation exactly as above. Alter the value of P slightly by inserting in series with it a short piece of German-silver wire. The only effect will be to shift somewhat the positions of C and C' along the scale, and thus the difference between R and S is obtained in terms of the length of a different part of the bridge-wire. If the wire be of uniform section the two lengths thus obtained will be the same. If they are not the same, it follows that the area of the cross-section, or the specific resistance of the wire, is different at different points, and a table of corrections can be formed as for a thermometer (p. 188).

If the difference between the two coils be accurately known we can determine from the observations the value of the resistance of a centimetre of the bridge-wire. This is given by equation (6); for the values of R-S and a'-a are known, and we have

For this purpose the following method is often convenient. Take two 1-ohm coils and place in multiple arc with one of them a 10-ohm coil. Let the equivalent resistance of this combination be R; then the value of R is 10/11 ohms. Instead of interchanging the coils place the ten in multiple arc with the other single ohm and make the observation as before; then in this case we have

and if l be the distance through which the jockey has been moved we obtain

Experiments.

(1) Calibrate the bridge-wire.

(2) Determine the average resistance of one centimetre of it.

(3) Determine accurately the difference between the resistance of the given coil and the standard i-ohm at the temperature of the room.

Enter results thus:

(1) Value of R-S for calibration, 0.009901 - being the difference between 1 ohm and 1 ohm with 100 in multiple arc -

(2) R-S = =.09091 ohm. l (mean of 5 observations) = 50.51 cm.

(3) Difference between the given coil and the standard at temperature of 15°C, observed three times.

Values 0.0037, 0.0036, 0.00372 ohm. Mean 0.00367 ohm.