This means, therefore, that when the electromotive force or potential is
great enough to send a current of 1/33 ampere through the body, fatal
results will ensue. This current is so minute that it would deposit only
about 6 _grains_ of copper in _one hour_, a grain being 1/7,000 of a
pound.
Let us now compare these figures with some actual cases, taking as
an example a system of incandescent lighting. In these systems the
difference of potential between any two points of the circuit outside of
the lamps does not exceed 150 volts. Taking this figure, therefore, it
will be seen that under no circumstances can the shock received from
touching these wires become dangerous--not even by touching the
terminals of the dynamo itself; because in neither case can a current be
driven through the body, sufficient to cause an excessive contraction of
the muscles.
In a system of arc lighting, however, we have to deal with entirely
different conditions; for, while in the incandescent system the adding
of a lamp, which diminishes the resistance, requires no increase of
electromotive force, the contrary is the case in the arc light system.
Here every additional lamp added to the circuit means an increase in
resistance, and consequent increase in electromotive force or potential.
Taking for example a well known system of arc lighting, we find that the
lamps require individually an electromotive force of 40 volts with a
current of 10 amperes. In other words, the difference in potential at
the two terminals of every such lamp is 40 volts. Consequently, if the
circuit were touched in two places, including between them only one
lamp, no injurious effects would ensue. If we touch the circuit so as
to include two lamps between us, the effect would be greater, since the
potential between those two points is 2 x 40 volts. We might continue
in this manner touching the circuit until we had included about 7 or
8 lamps, when the shock would become fatal, since the point would be
reached at which the difference of potential is great enough to send a
dangerous current through the body.
Up to this point we have assumed that, while touching two points in the
wire, the rest of the circuit is perfectly insulated, so that no current
can leak, in other words, that the circuit is nowhere "grounded." If
this is not the case we may, under suitable conditions, receive a
shock by touching only _one_ point of the wire. This becomes clear
by considering the current to leak from another spot of different
potential, to pass through the ground and into the body; thus, on
touching the wire the body virtually makes a connection between the
two points of the circuit. In clear dry weather such leaks are
insignificant; but in damp and rainy weather, and with poor insulation,
they may rise to such a point at which it would be dangerous to touch
the circuit even with one hand, the leaks being sometimes so great as
to cause the lamps to burn in a fitful, desultory manner, and to go out
entirely.
There is still another factor which enters into the discussion of the
danger of electric light wires. This must be looked for in the fact that
the physiological effects are greatest at the moment of the opening or
the closing of the circuit; or in a closed circuit they are the more
marked when the flow of current stops and starts, or diminishes and
increases. In dynamo electric machines the current is not absolutely
continuous or uniform, since the coils on the armature being separated a
distance cause a slight break or diminution of the current between each.
This break is so short that it does not interfere with the practical
work for lighting; in some constructions, nevertheless, the distances
apart is so great that, while not interfering with light, its effects
upon the muscles are greatly increased over those of other constructions
which give a more uniform current.
All these statements might lead to the conclusion that arc light wires
are dangerous under any circumstances; but this is not the case. The
first and only requisite is, that they be perfectly insulated. When thus
protected accidents from them are impossible, and all mishaps that have
occurred through them can be traced directly to the lack of insulation.
Nevertheless, we would warn our readers against experimenting upon arc
wires by actual trial, because unforeseen conditions might lead to
disagreeable results.
* * * * *