An experimental apparatus for illustrating the mutual attraction of
currents going in like direction. A cylindrical helix or spiral of wire
is suspended by one end. Its lower end just dips into a mercury cup. An
active circuit is connected, one terminal to the upper end, the other
terminal to the mercury cup, bringing the apparatus in series into the
circuit. The current as it passes causes the coil to shorten, each
spiral attracting its neighbors. This breaks the circuit by drawing the
lower end out of the mercury cup. The current being cut off the coils
cease to attract each other, and the end dips into the mercury cup
again. This closes the circuit, the coils again attract each other and
the same sequence follows and is repeated over and over again. A bright
spark is produced at each break of the mercury contact.