DIGI book chapter 4, Switches 4.1 under 'Liquid level switch' in the first sentence "an tank" should be "a tank".

 

Verified.

http://www.allaboutcircuits.com/vol_4/chpt_4/1.html

A floating object can be used to actuate a switch mechanism when the liquid level in an tank rises past a certain point. If the liquid is electrically conductive, the liquid itself can be used as a conductor to bridge between two metal probes inserted into the tank at the required depth. The conductivity technique is usually implemented with a special design of relay triggered by a small amount of current through the conductive liquid. In most cases it is impractical and dangerous to switch the full load current of the circuit through a liquid.

 

I will read it later.