Explosion-Proof Phone

Figure 1. The explosion-proof phone.

Excerpted from The Chronicle Vol. 64, No. 3, September 2011

by Patty MacLeish

This telephone, which weighs 44 pounds, was “picked up” by Jim Sellers last December (Figure 1). There is a Bureau of Mine Safety tag on the phone, which Jim learned was estab-lished in 1910. It’s not precisely a “Whatsits” as it is known what it is—an explosion-proof phone—but it is an interesting object and a bit of research provided insight into its design and use.

D.E.A. Charlton, assistant business manager, Engineering and Mining Journal Press, made a presentation at the 11th Annual Safety Congress of the National Safety Council on “The Use of Phones in Mines” that was later published in the Council’s Proceedings (pages 512-518).

Charlton said, “…not a shift passes but the usefulness of this invention is made manifest and it hardly seems necessary, in these days of so-called civilization and enlightenment that it is needful to enact laws that make the use of the telephone in mines compulsory.” The Federal Leasing Act of February 1920, he noted, required that each mine that employed more than one hundred men underground, required phones in specific areas and that men should not be more than one thousand feet from the nearest phone and that phones should be in all “refuge and first-aid chambers.”

Figure 2. The inner workings of the phone. A 1920 article describes if not this phone, a similar one, in detail.

Charlton also noted:

“The severe conditions imposed on mine telephones make it necessary to supply a more rugged construction than that required for ordinary installation. Underground, the device frequently must be exposed to moisture, gases, acid water and mechanical sources of injury. Of these, the latter, which include falls of ground, concussions of blasting and rough handling, is the most frequent; moisture is probably the next. gas and acid-water conditions are somewhat restricted to coal and copper mines.

“The generally accepted types of mine telephones are housed in a moisture and rust proof iron or steel case, of sufficient thickness to provide ample protection from injury. The edges of the case are well rounded so that water and falling object will easily slide off, and strong mounting supports are provided to insure rigidity when the telephone is installed. An outer door on the case, provided with a rubber gasket, serves, when closed, completely to protect the mechanism from any disturbing element. When this is opened only the transmitter, receiver, receiver cord and generator handle are exposed [Figure 1], as an inner door effectually conceals and protects all of the delicate mechanism [Figure 2]. When considered necessary, locks or padlocks may be placed on the outer door and keys provided to the shift bosses or foremen. Binding posts for the line and ground wires are placed in a terminal box, which is either mounted on the under side of the case proper or included as a part of it. The bells, bell mounting and clapper rod assembly are housed in a dome-shaped casting on the top of the case.

“The talking apparatus of the mine telephone consists of a standard long-distance transmitter and receiver, slightly modified for underground service. In the design of these, manufacturers have considered carefully the hard usage to which the mechanism will be subject and waterproof windings and special insulation are used throughout. The ordinary form of gravity-controlled hook switch is not sufficiently rugged for mine service and one manufacturer, at least, has devised a positive spring-controlled hook switch in which the force of gravity is not employed. The shaped yoke, normally rests between the jaws of a special receiver holder. When the receiver is taken out of the jaws a trigger depresses a small plunder which passed through the door and actuates the contact spring of the hook switch.”

Reported in the discussion section of the Proceedings were the remarks of a Mr. Parker who “stated that he was not in favor of the suggestion made in Mr. Charlton’s paper, that underground phones be locked and accessible only to a shift boss or other official who carried a key. Mr. Parker thought this was wrong and the telephone should be accessible to anyone in a case of emergency.”

Thank you, Mr. Parker! I am certain the miners of the day appreciated your sentiments.

The Proceedings are available online at google Books.