1222

THE COLLIERY GUARDIAN.

December 11, 1914.

Permissible Electric

By H. H.

The Bureau of Mines has investigated a number of
electric lamps submitted for the purpose of establishing
their safety and has approx ed three of them for use in
gaseous mines. The purpose of this paper is twofold :
First, to describe the methods followed in testing the
lamps for safety, and, second, to discuss other qualities
that portable electric lamps should have in order to be
acceptable for mine service.

The reason why the Bureau of Mines advocates the
use of portable electric lamps in mines is because fire
and explosion hazards will be decreased thereby.
Manifestly, therefore, the electric lamp itself must not
be a source of danger. The Bureau proved by actual
testj* that the glowing filaments of portable electric
lamps are capable of igniting mine gas, but that flashes
produced by equipments of not more than 6 volts are
not capable of igniting mine gas, unless the equipments
are larger than are likely to be used. When, therefore,
the Bureau decided to make tests to establish the
permissibility of lamps for use in gaseous mines, flashes
were dismissed as not being an element of danger,
whereas safeguards were required for the glowing
filaments. The Bureau’s approval of a lamp as
permissible under the conditions set forth means that
the Bureau vouches for the safety of the lamp but not
for its light-giving capacity, time of burning, or expense
and care required for maintenance. It was the purpose
of the tests to determine whether, under any conditions
whatever, gas could become ignited from the glowing
filaments of the lamps.

The Ceag Lamp.

The Ceag lamp is the first lamp that was approved
by the Bureau as permissible for use in gaseous mines.

Twenty-five tests were made to determine whether or
not the glass dome and the bulb could be broken
without interrupting the electric circuit of the lamp.
The lamps were tested by driving against the glass
dome of the lamp a tool shaped like a pick point. The
blows thus administered were sufficient to shatter both
the glass dome and the lamp bulb, but in 40 per cent,
of the trials the filament was not injured, a tact that
demonstrates both the remarkable strength of the
filaments and the necessity of safety devices. The
safety devices with which the lamp was provided
operated perfectly in each of the 25 tests, as evidenced
by the fact that the electric circuit of the lamp was
interrupted and that the gas-and-air mixture was not
ignited.

Eight tests were made to determine whether or not a
blow from the pick-pointed tool could jam the safety
devices so that they could not act properly. These
tests were similar to the previous ones, with the excep-
tion that the blow of the testing tool was directed
against the reflector e in an attempt to jam it against
the lamp socket. The safety devices were prevented
from acting in four of the eight trials, but in none of
these trials was the lamp bulb broken. Moreover, each
failure of the safety devices to act was due to the fact
that the lamp and testing tool were held in rigid
relation to one another and remained jammed together
after the blow was delivered. Such a condition could
scarcely exist in a mine, and therefore these failures
would probably have not been counted as such even if
the lamp bulb had been broken. The fact that the
lamp bulb was not injured made it unnecessary for the
safety devices to act, and as they did act whenever
their services were clearly required, their performance
was regarded as satisfactory.

Thirteen tests were made to determine whether or
not the safety devices acted quickly enough to prevent
the filament from igniting gas while the devices were in
operation. The glass dome was not used and the blow
of the testing tool was directed against the lamp bulb.
A wooden block- was wedged between the top of the
lamp frame and the top spring, so that the top spring
was held in its usual position. In six of the 10 tests
made the lamp filament was not injured, and in none
of the tests was the gas ignited.

Fifteen tests were made on the Ceag lamp to deter-
mine its general durability and the possibility of the
safety devices acting when such action was not
necessary or desirable. These tests consisted in
dropping the lamp 15 times upon a concrete floor from
a point 6 ft. above the floor. The initial position of the
lamp was varied so that different parts of the lamp
came in contact with the floor. The lamp was
extinguished in three of the 15 trials. In each instance
the jar of the blow shattered the glass dome and thus
caused the safety devices to act. The damp was
dropped five times before the battery was injured at all,
and 12 times before a permanent leak was made in the
battery jar. After the lamp had been dropped 15 times,
it continued to burn for 16 hours before going out
entirely.

The Hirsch Lamp.

The Hirsch lamp (fig. 1) is designed for cap service,
and was the second approved by the Bureau as
permissible for use in gaseous mines. The safety
devices with which this I mp is equipped are mounted
in the headpiece and consist of an open-circuiting
device that protects the lamp against blows from the
front, and a short-circuiting device that protects the
lamp against blows from the side. The open-circuiting
device is operated by the breaking of a slip of window

* From Technical Paper 75, U.S. Bureau of Mines.

t Clark, H. H., “ Ignition of gas by miniature electric
lamps with tungsten filaments/’ Technical Paper 23, Bureau
of Mines, 1912, 5 pp. ; “ Portable electric mine lamps ” :
Technical Paper 47, Bureau of Mines, 1913, 13 pp.

Lamps for Miners.*

CLARK.

glass a that is mounted directly across the inner surface
of the bull’s-eye glass b, with which the lamp is
provided. The breaking of this slip of glass releases a
spring c, that interrupts the electric circuit of the lamp.
The short-circuiting of the lamp is accomplished as
follows:—The headpiece is made up of three concentric
shells d,e, f, separated by narrow spaces. The outer
and inner shell are connected to the positive pole of the
battery, and the intermediate shell is attached to the

Fig. 1.—Hirsch Lamp.

6

c
dy
J

a

I
I

negative pole. These shells, of course, completely
surround the lamp bulb g, and the theory of the safety
device is that the bulb can not be broken without so
jamming these shells together that they will short-
circuit the battery and thus extinguish the filament
before it can ignite gas.

Forty-five tests were made upon this headpiece by
striking it with the following tools:—A hammer, a
wooden mallet, a tool shaped like a miner’s pick, a piece
of iron pipe, and a wooden club. Some of these tests
were made while the headpiece was rigidly supported in
various ways, and others were made while the head-
piece was swinging from the end of its cord. The
blows were struck with sufficient force to crush the
headpieces, to shatter the glass in almost every trial,
and to punch holes completely through the shells, but
every time that the lamp bulb was broken the safety
devices extinguished the filament. In some of the tests
the lamp was protected by the circuit breaker, and in
others by the short-circuiting device.

These tests were not made in gas. The extinction of
the filament was taken as the measure of safety, since
the filament, if extinguished at all, is extinguished
before the lamp bulb is broken, because the blow that
breaks the bulb must first destroy the slip of glass a or
jam the shells d, e,f. As a matter of fact, the bulb was
broken in only seven of the 45 tests made, although one
or the other of the safety devices acted in each test.

In order to find out whether or not the safety devices
would extinguish the lamp unnecessarily, the headpiece
and its cord were dropped 10 times upon a concrete
floor from a point 6 ft. above the floor. The safety
devices acted in only one test, and the action was
considered to be necessary, as the blow that tripped the
circuit breaker also shattered the outer glass of the
headpiece. These tests therefore seemed to prove that
the safety devices are so designed that they will not
cause the lamp to become extinguished unless it is
desirable that it shall be extinguished.

The hand-lamp feature of the Hiisch equipment was
disapproved.

The Wico Lamp.

The Wico lamp (fig. 2) is designed for cap service,
and was the third lamp approved by the Bureau as
permissible for use in gaseous mines. The safety
feature of this lamp consists in so mounting the bulb b
that it is held in its socket c by a wire stirrup, a, against

Fig. 2.—Wico Lamp.

the pressure of springs within the socket that act to
eject the bulb should the stirrup be removed or the
bulb broken. The tests made on this lamp were similar
to those made on the Ceag lamp, because the principle
of the protective devices was the same.

Qualities other than Safety.

The quality of safety, the most important a4tribute
of the portable electric lamp, has already been discussed,
but other qualities are desirable. The Bureau is at
present seeking to aid the development of portable
electric lamps by analysing the qualities that such
lamps should possess and suggesting a specification or
definition of what these qualities should be. Of course
different opinions will be held as to the merit and

desirability of this or that particular quality, but
probably all can agree upon the main heads under
which the various opinions can be grouped.

The first requisite of a lamp is the production of
light, and for mining service a lamp should burn
steadily and with undimmed brilliancy for a certain
number of hours of every day in the year. The next
requisite is lightness; that is, a lamp should not weigh
so much that it hampers a man’s movements or becomes
a burden to him. The next requisite is a cost of
operation and maintenance consistent with the work
done and the benefits received. There is another
important requisite that is so well recognised as not to
require discussion, and that is the prevention of the
leaking or spilling of electrolyte while a lamp is in use.

A true measure of the illuminating power of a
portable electric lamp must consider not only the
intensity of the light, or candle-power, but also the solid
angle over which the intensity is maintained. The
term flux ” is used by illuminating engineers to
designate the product of the intensity and the solid
angle of illumination, because this product represents
the light that flows out of a lamp. The unit of flux is
called a lumen and is about ths of the total flux of
light produced by a source or one spherical candle-
power. The term candle-power used without qualifica-
tion is not only confusing but meaningless. A lamp
that uses a reflector may have a “ head-on” candle-power
three to ten times the average candle-power over its
entire stream of light. Generally it is best to state the
average candle-power of a lamp instead of the candle-
power ata single point or group of points.

A statement of the candle-power of a lamp does not
sufficiently define its light-giving capacity. A 100-candle
power lamp is seemingly 33 times as desirable as a
3-candle power lamp, and yet a 100-candle power lamp
shining through a hole | in. in diameter gives less actual
light and much less useful light than a 3-candle power
lamp shining through a hole 3in. in diameter. There-
fore, in order properly to define the light-giving capacity
of a lamp a statement must be made regarding both the
candle-power and the total flux of light (or lumens)
produced by the lamp.

The selection of proper lower limits of intensity of
light and of flux of light is, aside from safety, the most
important thing in the consideration of portable electric
lamps.

It was finally decided to prepare a standard Wolf
safety lamp, as representing a type of modern safety
lamp extensively used in gaseous mines in this country,
to give its best performance and, after the flame height
had been adjusted to 1 in., to measure the average
intensity of the stream of light and also the total flux
of light in the stream. The lamp used was a Wolf
miner’s safety lamp, 1907 model, round burner, burning
naphtha having a specific gravity of 0*693 to 0*70, and
prepared and trimmed in accordance with the standard
practice of the Bureau of Mines. The average intensity
of light stream as determined by these tests was a
trifle under 0*4-candle power, and the total flux of light
was found to be not quite 3 lumens.

The Bureau suggests that lamps designed to be worn
upon the cap should give the same intensity of light as
that required for hand lamps, but that the minimum
flux of light required from cap lamps should be not
more than half the minimum demanded from hand
lamps, because when a lamp is worn upon the head any
light that is thrown to the rear is wasted. Therefore
the Bureau concluded that 1*5 lumens would be a
satisfactory lower limit for the flux of light produced
by a cap lamp.

Twelve hours was selected by the Bureau as a
reasonable time of burning. At least an hour of
burning should be allowed over and above the usual
requirements of the miner in order not to push the
battery too hard in an emergency and to allow for
possible incomplete charging.

The only interruptions of service that can be regarded
as reasonable are those incident to the renewal of such
parts of the equipment as have an admittedly limited
life, such as electrolyte, lamp bulbs, and battery plates.
The Bureau believes that it is not unreasonable to
require 300 hours of actual service from each lamp
bulb used with primary batteries or with acid storage
batteries, and 200 hours from lamp bulbs used with
alkaline storage batteries.

The most serious of the allowable interruptions of
service will be caused by the failure of the battery
plates. The Bureau believes that it is not unreasonable
to require of each battery plate of acid storage batteries
the equivalent of 3,600 hours of actual service in 12-hour
shifts, the service to be completed within one year from
date of beginning, and of each plate of alkaline storage
batteries 7,200 hours of actual service, to be completed
within two years from date of beginning.

Interruptions in the service of cap lamps are also to
be expected from the failure of the cord that connects
the battery with the headpiece. Although a reasonable
time-of-service requirement can be placed upon this
part of the equipment, there is no way other than
actual service to determine whether or not a cord meets
the requirement. That is, any tests would have to be
arbitrary and might or might not be too severe.

Interruptions of service other than those just
mentioned may be regarded as unreasonable and as
reflecting upon the design and construction of the
lamp equipment. Among such interruptions may be
mentioned those resulting from failures of terminals
and leads, battery jars, battery casings, and contacts,
as well as any failure of bulbs or plates before their
specified time of service has expired.

It is manifest that every time a lamp bulb, a battery
plate, or a cord is renewed, the cost of maintenance is
increased. Therefore, as regards both cost and service,
it is desirable that even reasonable interruptions of
service should be as few as possible.