Heurtley magnifier, 1919.
© Science Museum / Science & Society Picture Library
The Heurtley magnifier was a way of detecting weak telegraph signals in the pre-electronic era, and is based on the Wheatstone bridge developed by Charles Wheatstone (1802-1875) in 1843. When the bridge is balanced, a detector connected to it shows no response, but small changes in balance have a great effect. It used incoming telegraph signals pasing through an electromagnet to move hot wires in and out of a cooling air blast. The resulting changes in balance operated a relay, which switched the larger currents needed to drive the recording equipment. Shown out of its case.