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_.