HEDGEHOG
TRANSFORMERS

by Ric Havyatt


Transformers for the conversion of a voltage from one level to another have been in use since the days of the introduction of alternating current in the latter part of the 19th century. Transformers at that stage were of the power variety, i.e., to step up the voltage to a higher level for transmission over some distance, and a corresponding step-down transformer at the far end. For satisfactory operation of a transformer it is necessary to have an iron core over which the primary and secondary coils are wound.

James Swinbourne (1856-1958) is credited with the invention of the 'hedgehog' transformer where a bundle of iron wires formed the core of the transformer. Modern transformers now use flat steel laminations for the core, but hedgehog transformers were around for quite a while before being completely superseded. Windings for the transformer are wound onto a former with a suitable gap for insertion of the iron wires. The core wires are cut to such a length that their ends can be bent back to overlap and the whole device held together by straps to the wooden base. To prevent eddy current losses in the core, the wires are individually insulated.

The illustration shows a hedgehog transformer designed to work at audio frequencies, and this one would have been used for telephonic purposes. In 1914 Western Electric was working on a telephone repeater system for trunk lines to boost signal strength. The first long line system was put into operation in 1915 to provide communication from one side of USA to the other. The triode valve (tube), invented by De Forest, provided the amplification necessary and transformers were used for input and output of the repeater. A number of repeaters at regular intervals would have been involved in this long distance line.

The purpose of the particular transformer in this illustration is not known. It has three windings which are stated to be 302, 193 and 2 ohms respectively. Other transformers of similar construction are in the hands of collectors with completely different winding resistances, and ranging from just two windings to four windings. It is not known why winding resistances were stated for the transformers because turns ratio of the windings is more important in achieving correct circuit matching. Even when flat steel laminations had become the standard method of core construction, hedgehog construction was still used for the audio section of some radios. Atwater Kent (USA) in 1924 manufactured battery operated valve radios where the audio interstage transformers used hedgehog construction, even though most other manufacturers of that time had long since made the change to flat steel laminations

One application for these transformers would be for matching an unbalanced telephone line to a balanced line in the days when such single-wire systems were in use. A farm single-wire line to the road would require a matching transformer to connect to the two-wire system out on the main road. Speech currents and also ringing voltages are thus transferred from one system to the other without causing any problems due to earthing the main two-wire transmission system.

These transformers make an interesting and educational addition to any telephone collection.