Tangent Galvanometer

PHY 201 & 231

Students determine the local magnitude and direction of the earth's magnetic field. As a measurement tool, a galvanometer is constructed using the magnetic field produced by DC through a coil of wire to deflect the needle of a directional compass. Because the coil's field is set up perpendicular to the horizontal component of the earth's field, the two compete as vector components of the resultant along which the needle points. By calculating the theoretical coil field for a measured current and coil radius, as well as measuring the angle of needle deflection between the earth field and resultant vector sum of the earth and coil fields, the trigonometric tangent allows for a numeric solution for the earth field's horizontal component. Since a single solution is victim to a great amount of experimental error, numerous deflection angles are measured along with corresponding currents and the linear relationship between current and the tangent of the angle is plotted graphically. The slope of the best fit line established by all of the data in a linear least squares fit provides the numeric basis for a solution for the earth field's horizontal component, Finally, a special compass (dip needle) designed to rotate in a vertical plane is aligned with the sum resultant earth field providing the angle between the resultant and its horizontal component. This angle and the earth's horizontal component magnitude are then used along with the cosine function to solve for the magnitude of the resultant magnetic field of the earth.

Equipment

Notes

Handle apparatus gently. Put out the Extech power supplies. If there are not enough, use the Sargent-Welch power supplies. Put out the silver compasses with high precision gradings.