Theiler was an early telegraph engineer and instrument maker, who moved into telephones as well. He was a competent inventor and, from the quality of his telegraph work, a skilled machinist. He turned out a number of variations to the telephone, most of which have now lapsed into obscurity. Information on him is scarce, but this article summarises what is currently known about him and his phones in the hope that more information may turn up.
Theiler was born in 1845 in Switzerland. Little is know of his early years or his technical training. By 1881 he was living in London. The Census records that he was married and carrying on business as Meinrad Theiler and Sons, producing telegraph and telephone equipment. In 1883 the family retuned to Switzerland.
Most magnetodynamic telephones use the vibration of a diaphragm in a magnetic field to produce a varying current through the magnet coils. Meinrad Theiler's approach was to connect the diaphragm directly to one pole of the magnet and vibrate the magnet itself, something like a tuning fork. If the poles were sufficiently close together, the vibration would produce the varying current needed. It worked, and he was able to use the device both as a transmitter and receiver. It was inefficient as a transmitter, and he changed to a version of the carbon pencil microphones that were a popular way to avoid American Bell's patents.
Left: Intercom phone
Right: Exchange line telephone
Illustrations courtesy Ric Havyatt
Some his phones were brought into Australia in 1883, for evaluation by the NSW Electric Telegraph Department. These incomplete instruments have been unearthed and reconstructed by Ric Havyatt and Linley Wilson. Early receivers were also found in Melbourne, where Theiler phones were used on some Government private telephone lines. All known Theiler phones appear to have been made in England.
The transmitters on these phones are an early carbon pencil version, with two vertical carbon rods glued to the cork diaphragm. A horizontal square brass rod with a half round carbon face was suspended from the mounting frame by wires to rest lightly against the carbons, giving the variable contact needed. The arrangement was rather crude. The telephones known in Australia are a battery operated intercom set and a magneto telephone. In the 1883 Volume 12 of the Journal of The Society of Telegraph Engineers and Electricians, Professor Sylvanus Thompson notes that "Messrs Theiler have, to my knowledge, long employed metallic transmitters." The Society was discussing single-contact transmitters at the time, so it looks like Theiler had experimented with these as well
The early receivers were of the "spoon" or "wand" type, generally with an external ring magnet attached to a wooden shell. The two pole external ring magnet is quite distinctive, and appears to have been fitted to Theiler's earlier telephones. In a later version the magnet was enclosed in an ebonite shell inside the receiver, and was a single pole type.
The December 1885 issue of Scientific American outlines a version of Theiler's receiver. It used a horseshoe magnet, either permanently magnetized or with a field induced in the coils by an external battery. A light disk attached to one or both poles of the magnet served as a diaphragm, the movement of the magnet itself under the varying electromagnetic field being sufficient to move the diaphragm as the magnet's limbs attracted and repelled each other. It would have been rather inefficient and the article notes that "the patentees also employ two or more electro-magnets in the same circuit, and utilize the vibrations of both magnets in the manner described". In some versions the pole piece was hinged to allow more movement of the diaphragm.
The left diagram shows a two-diaphragm version. Note the short horizontal pieces attaching the diaphragms directly to the top of the poles of the magnet. The right version shows a pole piece fixed to the back of the case, while the single diaphragm is attached to the other pole.
The diaphragms were made of ivory, vulcanite, wood, or gutta-percha. They were designed to cover the entire ear opening, probably to increase their efficiency. For long lines, it was preferred to polarize a non-permanent magnet with power from a local battery, which could also power the transmitter. The line was connected to a secondary circuit in the coil. This allowed the use of more powerful batteries, improving the phone's efficiency even on weak line currents.
Preece and Maier in their 1889 book "Telephony" show a later "portable telephone". The receiver and transmitter are combined into a single unit, with a large horn to carry speech to the transmitter. The style is similar to the "cornet" used on many phones in France up until about 1920. The receiver uses a large permanent ring magnet, surrounding a coil which acts on the pole piece. This induces movement in the steel diaphragm - in fact, the receiver is now a standard Bell-type, modified to make it smaller. The Bell was now out of patent. The inside curve of the transmitter horn has a number of holes - C1 to C4 on the diagram - whose purpose is unknown, but Preece and Maier reported that they improved the quality of the transmitter.
For the transmitter, Theiler went to the proven carbon granule type, which was also out of patent. A fairly conventional construction of two diaphragms separated by a container of carbon granules, it was adjusted by an internal screw.
Theiler's phones were apparently never big sellers, and are now quite rare. Overall, their efficiency would have been fairly low, and it appears that they would have been used in an attempt to get around Bell's patents.
Good detailed pictures of a Dutch telephone from P J Kipp en Zonen equipped with a Blake transmitter and a Theiler receiver are available at the website of the Dutch Online Telephone Museum at http://www.telefoonmuseum.com
The Australian information gathered by Ric Havyatt and Linley Wilson is available from the website of the Australasian Telephone Collectors Association, at http://www.telephonecollecting.org/theiler.htm