This photo is from "Training Manual TM 2160-20 Submarine Mining," U.S. War Department, Washington, D.C., October 15, 1930, p. 76.

It shows a control panel for a group of 19 submarine mines. In modern terms, this would be called a rack-mount module, which was mounted on 6 foot-tall, 24 inch-wide iron frame, called an operating board (which we today would call a rack), together with a signal block and a terminal bar. One each of these components was needed to control a single group of mines.

Briefly put, the control panel mounted a selector (visible at upper left of the panel), which, in tandem with a similar device in the underwater distribution box, sent signals from the mine group indicating when a mine was tipped by a passing ship and back to the mines from the mine casemate indicating when an individual mine was to be detonated. The panel held a bank of 20 push-button switches for firing individual mines in the group. After a mine was fired, its switch could be depressed and rotated such that it remained depressed and could not be pressed again until it was reset.

The panel also mounted switching relays to put the mine group in or out of firing mode (a safety precaution), or to place the mines in the group on contact firing status, in which they would detonate automatically if they were tipped past an angle of 45 degrees from vertical.

Since the most busy mine casemate in Boston Harbor (for example) controlled 15 mine groups (285 mines), it would have mounted 15 of these mine control panels, plus many more related rack-mount devices for controlling the casemate's generators, inverters, and battery systems.

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