A great many so-called "high-resistance" telephones have recently appeared on the market for use with wireless detectors. Some of these have been found to be wound with German-silver wire. This gives the required resistance at a greatly reduced cost.

This fraud can be easily detected, even if the German silver is concealed under an outer layer of copper wire, as follows: Balance as nearly as possible on a sensitive Wheatstone bridge, so that the indicator comes to rest at or near the zero point. Then hold down the key for about a minute, allowing the battery current to flow through the apparatus. If ten bobbins of the telephone are wound with copper wire, the indicator will move slowly up the scale because of an increase in resistance, due to the heating effect of the current. With copper this change of resistance is considerable, while with German silver it is small.