A galvanometer is an instrument that can detect and measure small amounts of current in an electrical circuit. The first galvanometer was built just months after Hans Christian Ørsted demonstrated in 1820 that an electric current can deflect a magnetized needle. The device was assembled by German mathematician and physicist Johann Schweigger, who called it a multiplier. Essentially, a galvanometer consists of such a needle attached to a coil mounted so that the coil is allowed to pivot freely within a magnetic field created by the poles of one or more permanent magnets. When electricity is allowed to pass through the coil, the magnetic field generated by the current-carrying wire interacts with the field of the permanent magnets (traveling from north to south poles), generating a twisting force known as torquethat rotates the coil, a response elucidated by the left hand rule. The deflection of the galvanometer’s needle is proportional to the current flowing through the coil.
A simple, needleless galvanometer is presented in this tutorial. To observe the effect of an electric current on the coil, click the blue Turn On button to throw the Knife Switch. This action allows current from the Battery to flow through the circuit (from positive to negative), through the coil positioned between the opposing poles of two Bar Magnets. The magnetic field generated by the current in the coil is at a right angle to the plane of the coil; this field is depicted by the purple arrows, the tips of which denote the north end of the field. This induced magnetic field causes the coil to swing, as the south pole of the coil's magnetic field is attracted to the north pole of the bar magnet field. Once the coil is aligned in this manner it does not move again unless the direction of the current through the coil is reversed by clicking the Flip Battery button. This action reverses the poles of the magnetic field generated around the coil as the current passes through it, so the coil swings in the opposite direction. To stop the flow of electricity through the coil, click the red Turn Off button.