INDUCTION-COIL
You will remember that an electromagnet is made by winding many turns of wire around a piece of iron and sending a current of electricity through this wire.
Now, suppose this current of electricity was being supplied by two cells of a battery. If you took in your hands the wires coming from these two cells, giving, say, four volts, you could not feel any shock; but if you were to take hold of the ends of the wires on the electromagnet and separate them while this same current was going through, you would get a decided shock.
This separation would "break" the circuit, and the reason you would get a shock is that, while the electricity is acting on the wire, the iron itself is magnetized, and on breaking the circuit reacts upon the wire, producing for a moment more volts of pressure in every turn of it. Thus, you see, this weak pressure of electricity as it travels through the wire can yet produce, through its magnetism, strong momentary effects, but you cannot feel it unless you break the circuit.