WHEATSTON BRIDGE

EXPERIMENT 4： WHEATSTON BRIDGE

NAME STUDENT ID

ROOM：Fundamental teaching Building 1106

DATE: TABLE NUMBER:

PURPOSE:To experimentally verify the operation of a typical bridge circuit and tomeasure the resistance of an unknown resistor.

THEORY:

A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. It was invented by Samuel Hunter Christie in 1833 and improved and popularized by Sir Charles Wheatstone in 1843. One of the Wheatstone bridge's initial uses was for the purpose of soils analysis and comparison.

In its simplest form, the bridge circuit consists of four resistors, a battery and a sensitive galvanometer. The values of R1,R2 and R3 are all known, and Rx is the unknown resistance.

When the bridge is balanced by adjusting the standard resistance R2, until the galvanometer shows no deflection.(indicating no current flow through the galvanometer branch)Then points B and D in the circuit are at the same potential; current I1 flows through both R1 and Rx, and current I2 flows through both R3 and R2. Also, the voltage drop VAB across R1 is equal to the voltage drop across R3,VAD, for a zero galvanometer deflection:

Similarly,

By ohm's law

Hence, when the bridge is balanced, the unknown resistance Rx can be found in terms of the standard resistance R1,R2 and R3.

Equipment

rheostat, Double Pole Double Throw switch(which can change the current flow in the circuit), a galvanometer , resistancebox(0~9999.9Ω),wires.

PROCEDURE: 1. Connect the circuit shown in Figure 2. Take the DC power as 1 volts；Moving the point B near to the middle of the rheostat,is a protective resistor to preventexcessive currentdamage thegalvanometer.is a resistancebox. We adjust until the bridge balance.(Galvanometer point 0)But the problem is that R is a rheostat. We do not know the value of R1 and R2.Now let us consider thefunction of the switch.

3. When the switch is connected with AB,as shown in Figure 3, adjust R3 to let the Wheastone bridge balance. There will be：

Record the value of R3.

When the switch is connected with CD. As shown in Figure 4.The role of switch is exchanged positions Rx and R3.Then adjust R3 to let the Wheastone bridge balance again.In order to distinguish the R3 in Figure 3and Figure 4. The R3in Figure 4 we call it .

There will be：

We will get :

Note: Because the galvanometer is very sensitive, so we use a resistance box( ) to protect the galvanometer.

You should first adjust R’ to a high value to lower the bridge’s sensitivity, then adjust R3 to let the galvanometer return to zero.

Reduce R’ to a lower value to higher the bridge’s sensitivity, adjust R3 to let the galvanometer return to zero again.

Keep doing the uper steps, untilis zero and the galvanometer is zero too.

DATA REPORT：

R0 / / Rx

QUESTION：

  1. How to judge the Wheastone bridge is under balance state.

2.What’s the advantage of using the exchange method to get the unknown resistor?

1 / 4