Lab Report
Bsc Physics,1st years, Chennai Mathematical Institute
Carey Foster Bridge
Aim :
To determine the resistance per unit length, ρ of the Carey Foster bridge wire and hence to find the resistance of a given wire of unknown low resistance.
Apparatus :
• Carey Foster bridge • Two Equal Resistances • Thick Copper Strip • Fractional Resistance Box • Lead Accumulator • Galvanometer • Unknown low resistance • Key • Connecting Wires
Theory :
Circuit diagram for the experiment is shown in following Figure.
— The standard resistances P, Q. — Fractional resistance box X. — Unknown resistance Y. — A one meter long wire of uniform cross-section EF. — A lead accumulator with a key K . — Galvanometer G , Terminal B , Jockey D.
The position of jockey D is adjusted to locate the position where there is no deflection of the galvanometer when the jockey is pressed to make electrical contact with the wire; this position is called the balance point or null point. The bridge has its highest sensitivity when all four of the resistances, P, Q, X and Y, have similar magnitudes as the Carey Foster Bridge effectively works like a Wheatstone’s bridge If the balance point is located at a distance l1 from E, then we can write the condition of balance as : • P/Q = R/S = ( X + α + l1 ρ ) / { Y + β + ( 100 − l1 )ρ } … eq (1)
The Essay on Resistance Of Wire Length Electrons Doubles
A imI am going to be studying the resistance of wire. The purpose of this investigation is to see how the length of wire affect the dependent variable, resistance. Prediction I predict that, as the length of the wire doubles, the resistance will also double, This means that the length will affect the resistance. Hypothesis Resistance is caused by electrons bumping into ions. If the length of the ...
where α and β are the end corrections at the left and right ends. These end corrections include the resistances of the metal strips to which the wire is soldered, the contact resistances between the wire and the strips, and they also allow for the non-coincidence of the ends of the wire with the zero and one hundred division marks on the scale. Interchanging position of X & Y and combining with eq (1) and simplifying We get : • Y = X − (l2 − l1)ρ … eq(2)
where l2 is the new distance of the balance point from E. Once we know l1, l2, ρ and X, the unknown resistance Y can be determined .Clearly balance points will only be possible if the difference between the resistances, X – Y, is less than the total resistance of the one meter wire, (100 cm) ρ . To measure ρ I put Y=0 ie. a short circuit. Than ρ = X / (l2 – l1).
Procedure :
We perform the experiment in two steps. Part 1 : Measuring ρ of the Carey Foster bridge wire -1. We make the circuit connections as shown in Figure. In this part of the experiment Y is a copper strip that has negligible resistance and X is a fractional resistance box. To minimize any contact resistance between the terminals and the connecting wire We :(a) ensured that the wires and copper strip are clean and the terminals are screwed down tightly, and (b) close tightly all of the plugs in the resistance box . 2. We plug in the battery key so that a current flows through the bridge. 3. We press down the jockey so that the knife edge makes contact with the wire, and observe the galvanometer deflection. We release the jockey. 4. We move the jockey to different positions along the wire and repeat step 3 at each place until we locate the position of the null point, where there is no deflection of the galvanometer.We take care that the jockey is pressed down gently to avoid damaging the wire and distorting its cross section. 5. We note l1 using a table. 6. We reverse the connection and take readings.We average forward and reverse currents to minimize thermo emf’s and other random errors. 7. We take readings for different arrangements in Table 1. Part 2 : Determining unknown resistance Y -1. We remove the copper strip and insert the unknown low resistance in one of the outer gaps of the bridge. 2. We repeat the entire procedure as stated above and take readings in Table 2.
The Essay on Sc1 Resistance Wire Length Ammeter
Aim: In this investigation I want to find out how the length of a wire affects the resistance. Resistance: is a force that resists the flow of the current in a wire. Resistance = Voltage / Current R = V I Factors: The factors I think that will affect what happens in the investigation are: 1. The thickness of the wire. 2. The length of the wire. 3. Whether or not it is a good conductor. 4. The ...
Observations :
Table 1 : Determination of ρ for Carey Foster bridge wire -S no. X/Ω Position of balance point with copper strip in the l2 – l1 / cm ρ=X/ (l2 – l1) /Ω
right gap, l1 / cm : Direct 1 2 3 .2 .3 .4 39.65 34.38 29.22 Reverse Mean 39.51 34.5 29.21 39.58 34.44 29.21
left gap, l2 / cm : Direct 60.51 65.57 70.81 Reverse Mean 60.42 65.62 70.73 60.47 65.6 70.77 20.9 31.2 41.6 .0096 .0096 .0096
Table 2 : Determination of an unknown low resistance using a Carey Foster bridge. —
S X/Ω no.
Position of balance point with unknown resistance strip in the right gap, l1 / cm : Direct Reverse Mean 27.13 37.24 27.12 37.28 left gap, l2 / cm : Direct 72.91 62.68 Reverse Mean 72.87 62.76 72.89 62.72
l2 – Y = X + ρ ( l2 – l1 ) l1 /Ω / cm
1 2
1 1.2
27.11 37.31
45.8 25.4
1.44 1.44
Result :
1. The resistance per unit length of the bridge wire ρ = ______________ Ω m-1. 2. The value of the unknown low resistance Y= ______________ Ω 3. Error in ρ is ________ % & Y is ________ %.
