Wheatstone Bridge: Procedure

[ Wheatstone Bridge | Apparatus ]

Procedure: Assemble the circuit as shown in Fig. 2, omitting R_x for the time being. Ask your instructor to okay the circuit before you plug it in. For best sensitivity in each of the following measurements use the standard resistor R_s which is closer to the value of R_x , i.e. for 2. (see below) you expect the resistance of the voltmeter to be about 100 Ohms/Volt so you should use R_s = 300 W.

After placing R_X and the appropriate R_S in the circuit plug it in and adjust R₁ and R₂ to obtain a null reading. ( For best sensitivity keep R₁ and R₂ small; however do not allow the current through R₁ and R₂ to exceed the ratings posted on the bulletin board.) When you get close to a null reading, and only then press the tap switch to increase sensitivity.

• 1. Ask you instructor for an unknown resistor and determine its value.
• 2. Place the voltmeter in the R_X position and measure the resistance of the 3 Volt scale.
• 3. Place the thermistor assembly in the metal cup about half full of water. Measure the thermistor resistance. Heat the apparatus with an electric heater until the temperature is 60^o C. Shut off the heat and wait for the temperature to stop rising. When this happens (should be ~70^o C ) measure resistance of the thermistor. Allow the assembly to cool down, taking resistance readings every 5^o C, until the temperature is 20^o C.

The resistance of the thermistor in this range can be described by R=R_oe^a/T where R_oand a are constants and T is the temperature in ^oK. Make appropriate changes of variables to get a straight line and graph the results to determine R_o and a.

Note: Measure the resistance of the thermistor first at room temperature and then in the hot water. The resistance of thermistor should decrease when heated. If it does not, the thermistor is faulty and should be exchanged. Tag the defective thermistors.

[ Wheatstone Bridge | Apparatus | Procedure]