Varley's vacuum lightning protector.

Pole-suspended bare-wire telegraph lines are very vulnerable to lightning, which can damage the very delicate electrical instruments and possibly injure the operators. Lightning arresters provide a means of diverting lightning strokes to earth, and have been produced in many different designs since the beginnings of electric telegraphy. This one was designed by pioneer electrical engineer Cromwell Varley, and was acquired from the BT collection to help illustrate the great variety of designs.