The ballistic bridge, and another
A.C. Lynch · 2001
The Wheatstone bridge network, energised by DC and balanced by using a DC galvanometer, was described in 1843. Bridge circuits energised by AC, often using a telephone as detector, came into use chiefly after 1900. But there was a third possibility, the ballistic bridge, energised by switching DC either on or off, with a ballistic galvanometer as detector. At balance, the response of the galvanometer becomes zero, but it was recognised that this might mean either that no current had flowed through the galvanometer, or that there had been transient currents summed to zero. In the second case, the balance depended to some extent on the characteristics of the galvanometer. In 1886, Oliver Heaviside published a paper which at first sight appears to be a good account of the AC bridge, but it isn't. It is a study of ballistic bridges energised by rapid pulses both on and off, using the newly-invented telephone in the hope that it would be a sensitive detector. All ballistic bridge networks can be converted to AC networks, but there are AC networks which cannot become ballistic networks because there is no DC path. (4 pages)