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Calibration of a Bridgewire
The method gives us also the best means of calibrating a bridgewire. Make an observation exactly as above. Alter the value of P slightly by inserting in series with it a short piece of Germansilver 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 bridgewire. 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 crosssection, 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 bridgewire. This is given by equation (6); for the values of RS and a'a are known, and we have
For this purpose the following method is often convenient. Take two 1ohm coils and place in multiple arc with one of them a 10ohm 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 bridgewire. (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 iohm at the temperature of the room. Enter results thus: (1) Value of RS for calibration, 0.009901  being the difference between 1 ohm and 1 ohm with 100 in multiple arc 
(2) RS = =.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.


Home Electricity Calibration of a Bridgewire 