Measurement of small resistance by means of D.C. potentiometer.

Experiment No.: 05

Aim of the Experiment: –

Measurement of small resistance by D.C. potentiometer.

Apparatus Required: –

Sl. No	Name	Specification	Quantity
1	Standard cell	Weston type e.m.f. 1.0183 V	1 no.
2	Galvanometer		1 no.
3	Variable Power Supply (D.C)	(0-12) V	1 no.
4	Ammeter	(0-500) mA, PMMC	1 no.
5	Screw Gauge		1 no.
6	Connecting Wires	PVC insulated copper	As per required.

Table No. 5.1

Circuit Diagram: –

Theory: –

The potentiometer is of Crompton type. It is the simple slide wire potentiometer with some modifications. In this potentiometer a graduated slide wire is connected in series with eighteen coils each of which has a resistance exactly equal to that of the slide wire (of the order of 0.25 Ω). There are two moving contacts P₁ and P₂, sliding over the slide wire and studs of the resistance coils respectively. The supply battery (2 V), and R₁ and R₂ are two variable resistors. The former consists of some coils for coarse adjustment of the potentiometer current and the latter takes the form of a slide wire for fine adjustment as shown in Fig. 1. The galvanometer G is connected in series with a key K and a multiple circuit switch by which either the standard cells or e.m.f.s (voltage drops across standard and unknown resistances) to be measured can be connected in the galvanometer circuit.

Procedures: –

1. Make the connections as shown in the (fig.5.1) by the thick lines.

2. Standardize the potentiometer to make the potentiometer direct reading by adjustment of the current from the power supply as follows.

3. Weston type (e.m.f 1.0183 V) Standard cell is connected the terminal marked WESTON STD CELL (being sure to connect with the correct polarity). Press the button No.1 of the galvanometer. Resistances R₁ and R₂ are then adjusted until no deflection of the galvanometer is observed. Press the button No.2 of the galvanometer and obtain the balance. Leave the resistors R₁ and R₂ at settings so obtained.

4. Release the button No.2 of the galvanometer.

5. Switch over the multiple circuit switch to terminals of V₁ (voltage drop across standard resistor) V₂.

6. Switch over the multiple circuit switch to terminals V₂ (voltage drop across unknown resistance). Measure the voltage V₂ just like V₁.

7. Measure diameter and length of the conductor (Unknown Resistance).

Experimental Observation:

\[\color\green{Standard\; resistance}\;\color\red{ S\;=\;1Ω.}\] \[\color\green{Voltage \;drop\; across\; standard \;resistance}\] \[\color\red{=V₁}\] \[\color\green{Voltage\; drop \;across\; unknown\; resistance}\] \[\color\red{= V₂}\] \[\color\green{Unknown\; resistance,\; }\color\red{X =\dfrac{V₂}{V₁}\times{S}}\] \[\color\green{Diameter \;of\; the\; conductor\;}\color\red{ d\; =\; 0.87 mm}\] \[\color\green{Length\; of\; the\; conductor \;}\color\red{ l\; = \;1m}\] \[\color\green{So,\; the\; Specific\; resistance\; of \;the\; conductor}\] \[\color\magenta{p \;=\; \dfrac{Xd^2}{4l}\times{\pi}}\]

Observation Table: –

Sl. No	V₁ (Volt.)	I (Amp.)	V₂ (Volt.)	X=(V₁/V₂)S
1
2
3
4

Table No. 5.2

The value of the unknown resistance can be found out by finding the arithmetic mean of the R for different multipliers.

Precaution: –

Don’t push the key for prolonged state. The push button must be pressed momentarily to check if the pointer of the galvanometer is deflecting or not.
The terminal to which the apparatus under test is connected and marked positive (+) and negative (-) to avoid the possibility of damage to potentiometer due to wrong polarity being used.
Take many time between measurements of two voltage drops.

Conclusion: –

To be written by student.

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