Jefferson Lab's Workbench Projects
When power is supplied to the Electromagnet Experiment Stand, the green LEDs will light to indicate which settings are selected. Current will only flow through the electromagnet's coil while the momentary contact switch is pressed and the red power LED is on. As soon as this switch is released, no current flows through the coil and the red LED turns off. When 'FULL CURRENT' and 'ALL COILS' is selected, the complete electromagnet is connected directly to the 12 volt supply. As the coil has a resistance of about 2.6 ohms, it draws a current of about 2 amps. Selecting 'HALF COILS' removes the inner three layers of coils from the circuit. In order to keep the current draw constant, a 1.3 ohm dummy load is added in series with the remaining coils. Selecting 'HALF CURRENT' adds a 2.5 ohm dummy load in series with the magnet (either all six layers of coils or the outer three layers of coils and the 1.3 ohm dummy load, depending on which coil setting is selected), creating a total load of about 5 ohms and a total current draw of about 1 amp.
Typically, a number of paper clips are placed in a small container on the top of the base. The settings are selected and the power button is pressed. The magnet is not strong enough to pull the paper clips up from the base. The container of paper clips must be 'introduced' to the magnet by lifting it up with one hand while the other hand keeps the power button pressed. Different numbers of paper clips will be held by the electromagnet when different settings are selected.