The other day before today, we’ve discussed how we can measure small EMF by using a potentiometer and s standard cell. Initially, we find the approximate value of current eg. by using the required voltage of thermocouple. After we have obtained the value for R1 and R2 can we accurately measure the current in the circuit when the circuit is balanced (ie. the galvanometers are zero.)
Now, let’s have a look at Wheatstone Bridge, which is a resistive bridge. It is used to measure resistance, capacitance, and inductance.
The diagram on the left is a Wheatstone Bridge circuit. Compared to potentiometer, a Wheatstone bridge is very capable of high precision measurement of resistance, capacitance and inductance. Two things stand out: there is no need to determine the standard cell’s EMF and there is no need to calibrate the galvanometer.
W.B. makes use of 3 known resistors and 1 unknown resistor (the one we’re looking to find the value.) . We’ll do an exercise today…nope, not running…but I’m sure this one’ll get your mind running…
The Wheatstone bridge shown above is balanced when P = 10 ohm, Q = 50 ohm, R = 30 ohm and X = 1050 ohm. The resistor S consists of a 3.5 m length of nichrome wire of cross-section area 1.5 x 10-8 m2. Calculate the resistance of S, and the resistivity of nichrome. (No, the value of V is not needed for this calculation).
Or, you could always visit Walter Fendt’s excellent java applets on his webpage at http://www.walter-fendt.de/ph14e/. But then, I already told you guys about that.
