A coherer is a bistable device based on metal filings loosely confined between solid metal electrodes. This granular material normally exhibits a very high electrical resistance (tens of kilohms), but passage of the high-frequency current generated by reception of a radio signal causes it to "cohere" into a comparatively low resistance condition (tens of ohms). This state persists until the device is mechanically disturbed, whereupon the high resistance state is restored. This characteristic was employed by scientists in the 1890s to detect radio waves, and applied commercially by Marconi in his "wireless" telegraph. It is easy to make a working coherer and directions are given for operating it from a distance with a spark transmitter based on a piezoelectric gas igniter. Incorporation of an "aerial" and "earth" enable a range of 7 m to be achieved and simple signals may be transmitted. (Contains 6 figures and 2 footnotes.)