The Solenoid Valve: An Electromagnet That Opens a Pipe

A relay lets a small current switch a large one. A solenoid valve does the same for a fluid: a coil round a tube pulls a soft-iron plunger that opens or closes an orifice, so a switch, a thermostat or a timer can turn water, air, gas or oil on and off without a hand on the valve. Washing machines, irrigation timers, compressors and boilers all use them. The Automatic Switch Company (ASCO) of New York is credited with making the first solenoid valves in 1910. This rung works out why a solenoid's pull depends so steeply on its air gap, why that limits a direct-acting valve's orifice, and how a pilot valve gets round it — then drives a real 12 V valve from a switch and measures when it will and will not open.