Natural Philosophy Collection

The Speaking Arc

Our picture shows a large, brown, sealed box cracked with age, its top dotted with cryptically marked terminals set around a short metal pole; the whole heavy with hidden wires and hardened wax. Is it a venerable relic or just a piece of junk? The device was central to a remarkable advance made in wireless technology before the days of the first radio valve. In 1903 with a box not unlike this, a hydrogen arc discharge lamp, and little else, the Danish electrician Valdemar Poulsen cracked the single most important problem in radio transmission: how to generate a radio signal of one frequency only, from a constant voltage. The solution to this problem allowed many radio stations to operate in the same part of the world without mutual interference, each on its own frequency. It also opened the way for radio signals to carry undistorted speech and music rather than the Morse code they had previously been limited to. Radio telephony was thereby developed several years before the proper tool for the job, the radio transmitting valve, became available. What may have been the first public demonstration of wireless telephony in Aberdeen was given at Marischal College in 1911 but it is not known whether our speaking arc by Marconi's Wireless Telegraph Co. Ltd. was used on that occasion.

John S. Reid