This can best be described as a magnetic switch. It comprises a small solenoid coil around a sleeve and an iron core. Inside the sleeve is a plunger to which the switch contact bridge is attached; the contacts are normally open.
When the coil is energized, a strong magnetic field of force is created because the current will be high during the starting phase. The magnetic force will move the plunger upwards and bridge the switch contacts, completing the circuit to the start winding. The run winding is wired through the relay so that it is always in circuit. Figure 34 shows a typical motor and control arrangement for a current relay, and Figure 35 shows the completed relay circuit.
A high starting current is drawn when the compressor motor starts. The current reduces as the motor gathers speed; the magnetic field through the relay then becomes weaker so that it can no longer hold the contact bridge on to the switch contacts. The plunger then drops down by gravity to open the circuit to the start winding.
It is not uncommon for a start capacitor (see later) to be fitted when a current relay is employed. This is wired in series with the start winding (Figure 36).