How did the first electrical fuel gauge, introduced in 1925, require the driver to check the reading?

Answer

The driver had to press a button to momentarily close the circuit

The 1925 electrical iteration demanded active participation; the operator had to press a button which closed the circuit temporarily, allowing the indicator to display the fuel level only while depressed.

How did the first electrical fuel gauge, introduced in 1925, require the driver to check the reading?
inventionVehiclemeasurementtechnologyfuel gauge