Exp ( ) Calibrating of a sensitive galvanometer for measuring a resistance (Ohmmeter). Aims: • Calibrating of a sensitive galvanometer for measuring a resistance. The theory When a galvanometer is used as an ohmmeter for measuring an ohmic resistance R, the deviation angle θ of the galvanometer’s coil is directly proportional to the flowing current through the coil and inversely proportional to the value of the resistance. The deviation will reach to the maximum end when the resistance equals to zero or the current has the maximum value. For this reason, the scale will be divide by inversely way in comparison with the ammeter and the voltmeter. The original circuit as shown in Fig(1) consists of a dry cell(E) , rheostat and ammeter (A) are connected in series with small resistor s has the range of 1 omega. The two terminals of “s” are connected in parallel to another combination includes a sensitive galvanometer “G” which has internal resistance “r” and a resistors box “R”, this combination is called the measuring circuit. When the value of R equals zero, and by moving the rheostat in the original circuit, it is possible to set the flowing electric current in measuring the circuit as a maximum value od deviation in the galvanometer. By assigning different values of R, the deviation θ is decreasing with increasing the value of R or by another meaning when the flowing current through the galvanometer decreases. Therefore, the current through the galvanometer is inversely proportional to the value of R according the following Figure 1: Ohmmeter Circuit diagram equation; V=I(R+r) R=V/I-r or R=V/θ-r 1 | P a g e This is a straight-line equation between R and (1/θ) as shown in Fig(2). which can be used for determining the value of unknown resistance R depending on the deviation angle θ of the galvanometer. Figure 2: calibration curve Procedure: 1- Connect the circuit as shown in Fig (1). 2- In the absence of a resistance from the resistors box (R=0). Adjust the rheostat to achieve the maximum deviation θ of the galvanometer’s pointer. Then record the reading of θ. 3- Take different values from the resistors box R and record every time the deviation θ. Keep the value of the current constant in the original circuit during the experiment. 4- Tabulate the measured data in table No.1 5- Plot the relation between the resistance R (on the horizontal axis) and (1/θ) (on the vertical axis). 6- From the intercept of the straight line with R axis, determine the internal resistance of the galvanometer r. 7- Use the same relation to find unknown resistor if it will be available. 8- It is possible to prove the trueness of the determined r by taking different values from the resistors box until achieving a half of the maximum deviation. Register the resistance which equals the value of r directly. R θ 1/θ 2 | P a g e .