How capacitance works

Image: Capacitance

An electrical property of long cables Plates from inside a capacitance box, made from sheets of tinfoil interleaved with waxed paper and sealed in paraffin wax. This example is approximately 40 centimetres wide. Capacitors store an electric charge. When a voltage is applied across a capacitor, charge builds up on the plates until the voltage across the plates equals the voltage applied.

The circuit diagram for a capacitor. The two vertical bars represent the conductors and the space between them represents the dielectric.

A capacitor (also known as a condenser) usually consists of two conductors separated by an insulator, the dielectric. In the case of a telegraph cable, the insulation forms a dielectric between the cable and the sea water outside and the cable itself stores a charge. Over a long stretch a cable may have significant capacitance, which will slow down the speed with which signals can be transmitted.

When a current is applied to a line with capacitance and resistance, it takes time for the line to charge up. The longer the line, the greater its capacitance and the longer it takes to charge up. This is known as the CR time constant. This rate of charge slows down the speed at which each morse element can be transmitted.

In an ideal line with virtually no capacitance or resistance (eg a very short one) the output to a siphon recorder should look like this:

But the CR time constant slows down the rate of change from one signal to another, resulting in an output like this: