s Magnetic Field
5
Experiment 1
Field
Objectives:
as
a
t
Apparatus:
1. Tangent galvanometer
2. Digital multimeter
3. Rheostat
4. DC Power supply
5. Compass
Th
20 a
21 mm
Theory:
A tangent galvanometer consists of a current-carrying coil (in a vertical plane)
producing a horizontal magnetic field perpendicular to the horizontal component of the
indicated by a compass needle. The current in the
coil produces a magnetic field along the axis of the coil with a magnitude given by
BC
------------------(1)
BC = magnetic field at the center of the coil in tesla (T),
N = number of turns of the coil,
0 = permeability of free space in tesla.meter /ampere (T.m/A),
= 4 x10 7 T.m/A
I = current in ampere (A),
R = radius of the coil in meter (m).
ys
ic
s
where
NI
2R
0
Ph
The field due to the coil causes the compass needle to deflect from the north south
direction. When the compass needle has come to rest, it points in the direction of the resultant
of
magnetic
tic
field is , as shown in Fig.1, then
BC
tan =
BE
or
BC = BE tan
------------------(2)
6
Physics Laboratory II
t
Fig.1 The direction of the resultant magnetic field. All the fields are in the horizontal plane.
where
Substituting
2B E R
0N
Th
20 a
21 mm
From equation (1) and (2) we obtain
0 NI
= B E tan
2R
2B E R
or
I =
tan
0N
as
a
BE
B R = resultant magnetic field,
BC = magnetic field produced by the current in the coil.
-------------------(3)
= K = coefficient of galvanometer
I = K tan .
-------------------(4)
K 0N
BE =
and
-------------------(5)
2R
From equation (4), the graph of the current in the coil (I) against the tangent of the
deflecting angle (tan ) is a straight line with slope equal to K, as shown in Fig.2. Then the
Ph
equation (5).
ys
ic
s
thus
I (A)
slope = K
0
tan
Fig.2 The graph of current (I) vs. tan .
s Magnetic Field
7
Notes:
Th
20 a
21 mm
as
a
t
Experiment:
1. The tangent galvanometer is first leveled so that the pointer can move as freely as
possible and is then tested
coil should make an angle of 900 with the pointer. The compass needle points in the northsouth direction and should lie in the plane of the coil as shown in Fig.3. Connect the circuit as
shown in Fig.4 using the 50-turn coil (choose the appropriate connecting posts on the
galvanometer base). Put all magnetic materials such as metal pencils, watches, currentcarrying wires, power supply, etc., as far from the galvanometer as possible.
2. Turn on the power supply, adjust the voltage to 10 volts, and adjust the rheostat
until the ammeter reading is as close to zero as possible. Successively adjust the rheostat to
allow current to flow through the 50-turn coil so that the compass needle (or the pointer)
deflects, each time, by 10 degrees from its previous direction. Each time record the
corresponding current reading in Table 1 of the data sheet.
3. Repeat the procedure in 2 by reversing the current direction in the coil of the
tangent galvanometer so that the average current(I) can be obtained.
4. Repeat the above procedures from 2 to 3 but use the 500-turn coil instead (adjust the
power supply to be 5 volts). Record the results in Table 2.
5. Plot a graph of the average current (I) against tan from the results in Table 1 and
Table 2. Plot the data on the same set of axes, using I as the y- axis and tan as the x- axis.
6. Compute BE (in units of tesla and gauss) from both graphs and compare the results.
Ph
ys
ic
s
1. BE can be obtained from the slope (K) of the graph by substituting K in equation (5).
2. The magnetic field of intensity 1 tesla (T) = 104 gauss (G).
3. The standard value of BE = 0.361 gauss (G).
8
Physics Laboratory II
N
Plane of the coil producing
the magnetic field B c
Compass needle
as
a
t
Pointer
Th
20 a
21 mm
Angular scale
Fig.3 The orientation of tangent galvanometer. The plane of the coil is aligned with
the north-south direction as indicated by the compass needle.
Tangent galvanometer
Ph
ys
ic
s
Digital multimeter
(Ammeter)
Power supply
Rheostat
Fig.4 The connection between components.