Nick Hilditch, editor, Electrical Tester
Anyone who has done much work in the field of earth electrode resistance testing will know at least a little about driving earth spikes into the ground, and concepts like the concentric earth shells that surround the electrodes. They may be slightly surprised, however, to discover that those who were unfortunate enough to be fighting in the trenches a century ago during the First World War also had good reason to be familiar with these ideas.
They weren’t of course, particularly interested in making earth resistance measurements – they had somewhat more pressing matters on their minds. But they were interested in finding a solution to a challenging problem, that of how to communicate reliably with each other and with their commanders. To appreciate the magnitude of this challenge, it’s important to remember that at the time of the First World War, radio was primitive.
Spark transmitters were still in use and valves (vacuum tubes) were in their infancy. Lee de Forest had invented the triode, the first electronic amplifying device, just eight years before the war began, and the types available during the war were not only fragile but also temperamental and limited in performance. Solid-state devices, other than point-contact (cat’s whisker) diodes, weren’t even a glimmer on the horizon.
For these reasons, the main method of communication in the trenches was by wired telephones. These had, however, a rather serious and obvious weakness – the telephone wires were very easily damaged by shellfire so communications would frequently be interrupted, especially in battle situations where they were most needed.
One solution that proved surprisingly effective and was, as result used extensively, was the delivery of messages by carrier pigeons. Despite its benefits, however, this method is not particularly well suited to use during heavy artillery bombardments or for providing minute-by-minute situation updates. So another method was pressed into service – ground telegraphy.
Essentially, this involves driving two earth stakes into the ground as far apart as possible at the transmitting station, and connecting them to a circuit that includes a buzzer, a battery and a telegraph key (a type of switch that can be rapidly and repeatedly opened and closed). At the receiving station, two similar stakes are driven into the ground and are connected to a sensitive pair of headphones.
Via the earth stakes at the transmitting station, the buzzer injects rough AC currents into the ground. The receiving earth stakes pick up a tiny fraction of these currents which, according to reports published at the time, produced a signal that could be clearly heard in the headphones. With this arrangement, someone operating the telegraph key at the transmitting station could send a message in Morse code to the receiving station.
The ground telegraph had big advantages over other communication methods: it needed no wires between the transmitting and receiving station, and it needed neither delicate valves nor bulky power-hungry spark transmitters. One of its potential disadvantages, however, was that in principle all that the opposing forces had to do to listen in on the messages was to deploy a few earth stakes of their own. A partial solution was apparently found by using tuned buzzers, but the reports of the day are unsurprisingly rather cagey about providing much in the way of detail.
It seems that the ground telegraph achieved significant success, and it is reported as being widely used especially in the later phases of the war. If you’d like to find out more, the January 1918 issue of the American publication “Electrical Experimenter” contains an interesting article “Ground Telegraphy in War” and can be found on line at http://electricalexperimenter.com/n9electricalexperi05gern.pdf.
And next time you’re driving in those ground stakes to carry out an earth electrode resistance test, you might like to spare a thought for your predecessors who were doing essentially the same thing but for a rather different reason in the hellish conditions of the World War I battlefields.