In this age of electricity and electric wires carrying currents of
various intensity, the question of danger arising from contact with them
has caused considerable discussion. An examination into the facts as
they exist may therefore enlighten some who are at present in the dark.
To begin with, we often hear the question asked--why is it that certain
wires carrying very large currents give very little shock, whereas
others, with very small currents, may prove fatal to those coming in
contact with them? The answer to this is--that the shock a person
experiences does not depend upon the current _flowing in the wires_, but
upon the current _diverted from them_ and _flowing through the body_.
The muscular contraction due to a galvanic current, which was first
observed in the frog, gives a good illustration of the fact that it
requires only a very minute current to flow through the muscles in order
to contract them. Thus the simple contact of pieces of zinc and copper
with the nerves generated current sufficient to excite the muscles--a
current which would require a delicate galvanometer for its detection.
What is true of the muscles of the frog holds good also for the human
muscles; they too are very susceptible to the passage of a current.
In order to determine the current which proves fatal we need only to
apply the formula which expresses Ohm's law, viz., C=E/R, or the current
(ampere) equals the electromotive force (volt) divided by the resistance
(ohm).
According to the committee of Parliament investigation, the
electromotive force dangerous to life is about 300 volts; this then is
the quantity, E, in the formula. There remains now only to determine the
resistance in ohms which the body offers to the passage of the current.
In order to obtain this, a series of measurements under different
conditions were made. On account of the nature of the experiment a high
resistance Thomson reflecting galvanometer was used, with the following
results.
When the hands had been wiped perfectly dry, the resistance of the body
was about 30,000 ohms; with the hands perspiring ordinarily it fell to
10,000 ohms; whereas when they were dripping wet it was as low as 7,000
ohms. Our readers can judge this resistance best when we state that the
Atlantic cable offers a resistance of 8,000 ohms.
Taking an ordinary condition of the body, with the hands perspiring as
usual, we would have the resistance equal to 10,000 ohms. Applying the
two known quantities in the formula, we get: