1851 Great Exhibition: Official Catalogue: Class X.: Thomas Allan
201. ALLAN, THOMAS, 20 St. Andrew's Square, Edinburgh — Inventor.
Two pairs of patent electric telegraphs.
[The general principle upon which electric telegraphs depend for their indications, is the remarkable fact discovered by Professor Oersted, that a magnetic needle freely moving on its axis is capable of being turned to one side by the transmission of an electric current through a wire placed parallel and near to it. It was subsequently found, that by placing the needle so as 3 to surround it with a coil composed of many lengths of insulated wire, this effect was immensely multiplied, and a very feeble current became sufficient to deflect the needle so placed. The galvanometer was thus invented, and subsequently the needle-telegraph. It is obvious that a means of communicating signals was discovered when this fact was first developed, since a needle thus placed might be arranged at any distance, and being connected with the operator by insulated wires, he could cause it to turn to one side at pleasure. Such is the principal feature of the needle-telegraphs. These instruments consist essentially of the following parts — a source of the electric current, or voltaic battery, a medium through which it can be conveyed without loss, or insulated wires, and a magnetic needle arranged so as to be influenced by its passage, which is generally suspended in front of the index-plate of the apparatus, upon which certain marks are arranged. The movements of two such needles, and the combination of signals which these obviously afford, form the alphabet, or signal code of the electric telegraph.—R. E.]