Physics Lab

Topics: Refraction, Snell's law, Refractive index Pages: 38 (4389 words) Published: February 28, 2013


In this experiment we will measure the magnitude of the horizontal component of the Earth's Magnetic field by the use of an instrument called a tangent galvanometer.


A tangent galvanometer consists of a number of turns of copper wire wound on a hoop. At the center of the hoop a compass is mounted. When a direct current flows through the wires, a magnetic field is induced in the space surrounding the loops of wire. This magnetic flux is designated by Bi . The strength of the magnetic field induced by the current at the center of the loops of wire is given by Amperes law:

Induced Bi = [pic].

where μo is the permeability of free space and has the value of 4π x 10-7 N/A2, N is the number of turns of wire, I is the current through the wire, and R is the radius of the loop.
When the wire loops of the tangent galvanometer are aligned with the magnetic field of the Earth, and a current is sent through the wire loops, then the compass needle will align with the vector sum of the field of the Earth and the induced field as shown in Figure 1.


Fig. 1

The horizontal component of the magnetic field of the Earth is easily calculated from the following relation:

B of Earth = [pic].


Tangent galvanometerAmmeterLeads & connectors
Reversing switchRulerRheostat, 20 Ω
DC supply, 6 VPlywood board


1.Set up the apparatus on a board between tables as shown in Figure 2. Be sure to orient the loops exactly in the North-South direction. Orient the compass so that the needle is pointing to zero degrees.

|[pic] | | | | | | | | |Fig. 2: Apparatus Wiring Diagram |[pic] | | | | | |Binding posts configuration |

2.Supply power to the 10-turns binding posts and adjust the rheostat until a deflection of 45o is indicated on the compass. Reverse the current to obtain a 45o deflection on the other side of the compass. Record the exact current for each deflection.

3.Sketch a vector diagram for the situation where there is a 45o deflection. Calculate the magnitude of the horizontal component of the Earth's magnetic field. The SI unit for B is the Tesla (T). There are 104 gauss per Tesla.

4.Repeat steps 2 and 3 for a 63.5o deflection. What is the relationship between the Earth's field and the field of the loop for this case? Draw a vector diagram.

5.Repeat the entire procedure for the 15-turns binding posts. What conclusion can you draw about the magnetic field of the loop from this part of the experiment?

DATA SHEET: The Tangent Galvanometer

Data and Calculations table for 45o deflection

| | |Current | | | |Number | |(A) |Binduced |BEarth | |Of Turns |Deflection | |(T)...
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