General Industries Carbon Microphone

This double button carbon microphone was made by General Industries in the late 1920's.  It is a large unit, measuring about 3 3/4 inches in diameter (not counting the spring hooks), and just over 1 1/2 inches thick.  It is made from aluminum castings, so it is relatively light, at 1 1/2 lbs.  It was advertised to have a frequency response of 30-7000 Hz, and was suitable for broadcast use.  It was priced at about $70, so it was an expensive unit.

The rear of the microphone has three concentric rings of screws.  The outer ring (12 screws) holds the two halves of the microphone together.  The two inner rings (8 and 4 screws) appear to be adjustments by which the diaphragm is tensioned to give the best frequency response.
This microphone was marketed by E. F. Johnson as seen in this ad which appeared in March, 1929.  Founded in 1923, E. F. Johnson started out as a mail-order distributor of radio parts and equipment.  They grew to be a major manufacturer of high quality communications equipment and components for the professional and amateur user.  Today, their focus is communications equipment and systems  for law enforcement, fire-fighting, EMS, and the military.
This picture of a portly gentleman was found along with the pictures of a late thirties audio equipment shop.  Very prominent in this picture is a General Industries carbon microphone on a desk stand,  and to its left, an Automatic Electric model 1A telephone.  My guess is that the photo dates to the early thirties.  Directly to the left of his head is an advertising display of some sort, but there is no printing visible on it to hint at it's location.   From the context of the other pictures in that group, this would have to be a public address system, and this must be a manager's office.  The advertising piece would suggest that this was a store of some sort.  Click on the picture for a larger version.