Igranic Pentrovol

Igranic Electric Co. Ltd. was established in Bedford, England, in 1906, first focusing on motors and electrical parts. By the 1920s, the company had expanded into manufacturing radio equipment and components.

From Wireless World, February 1929

"On the Igranic stand considerable interest was displayed in the new "transverse current" microphone. This instrument has an excellent response curve and has been designed for broadcasting studios and public address systems. The carbon granules are of graduated size and are segregated into a number of cells, each cell being designed to deal with a particular band of frequencies. In this way it has been found possible to produce a better response characteristic than would be otherwise obtainable. The energising current is passed transversely through the cells in series and not from back to front as in the conventional carbon microphone.carbon granules A neat control unit is associated with the microphone in which the switching has been reduced to the simplest possible form."

This microphone has 'Pat No 301441' marked on both the front and the rear, which refers to British Patent GB301441, which was applied for in August 1927 by Leslie Harold Paddle, and granted in November 1928.

The novel part of this patent is the use of five different chambers inside the microphone each with different sizes of carbon granules, all connected in series. This purportedly reduced specific resonant peaks which may occur if a single size of granule is used.

A contemporary advertisement identifies this as the Pentrovol model from the British company Igranic, which presumably refers to the five chambers. This is slightly confusing as the grill is divided into six sections - perhaps each divider straddles a carbon chamber.

The Pentrovol is very heavy and consists of a large brass barrel with a steel backplate and bakelite grills. The electrical connections are made via two screw terminals on the rear, which is a common approach for early carbon microphones. Four plated brass spindles are screwed to the back plate and these would have been used as anchors for the spring supports, which are no longer present in our example. An example with the spring mount is here.

British Patent GB301441

An Iganic Pentrovol in excellent original condition complete with spring mount is shown here at the RTP virtual museum, including a contemporary sound recording.