Low Resistance ( Carey-foster-bridge)
Item Code : SK303
Principle and Working:
It is the practical modified form of wheatstone bridge to increase the accuracy and sensitivity of the bridge for the measurement of low resistance.
The apparatus consists of manganin or constantan wire about 100cm in length, having uniform area of crossection, fixed by the side of metre scale. The ends of wire are soldered to the thick metal (copper) strips. Three more metal strips are fixed parallel to manganin wire, to make four air gaps. Two small resistances of the same order whose difference is to be determined are joined in the outer gaps in series with the bridge wire. Two equal resistances are connected in the inner gaps. The galvanometer, key, cell and jockey are connected as shown.
Formula used :-
1. Resistance of unit length of bridge wire.
6 = r / l2‘- l11
Where r = resistance drawn from resistance box and l1‘ and l2‘ are balancing lengths when resistance box is inserted respectively in left and right gap.
Unknown resistance Y = R – ( l2 – l1 ) 6 where
R = known resistance from resistance box and l1 and l2 are balancing lengths when resistance box is connected in left and right gap respectively
Read More
- Standard power supply 2V, 100mA.
- To calibrate the wire of Carey-Foster-Bridge and hence determine the value of given low resistance.
| S. No. | Item Name | Qty. |
| 1 | Carey-Foster-bridge with Jockey | 1 |
| 2 | Resistance module 10 ohm | 2 |
| 3 | Resistance modules 0.5,1,1.5 & 2.5Ω (each) | 1 |
| 4 | Sensitive galvanometer 20 μA/Div. | 1 |
| 5 | Thick brass strip | 1 |
| 6 | Power supply 2V/100mA | 1 |
| 7 | Connecting leads (red & black) 50cm (pair) | 1 |
| 8 | Connecting leads black 25cm | 10 |
| 8 | Unknown low resistances (approx. 0.22 & 2Ω) | 2 |
| 9 | Instruction manual | 1 |
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