"The machine is connected by wires to each of you, and will make
what are called electrocardiographs, in which every emotion, every
sentiment, every passion is recorded inevitably, inexorably. For,
the electric current that passes from each of you to the machine
over these wires carrying the record of the secrets of your hearts
is one of the feeblest currents known to science. Yet it can be
caught and measured. The dynamo which generates this current is
not a huge affair of steel castings and endless windings of copper
wire. It is merely the heart of the sitter.
"The heart makes only one three-thousandth of a volt of
electricity at each beat. It would take thousands of hearts to
light one electric light, hundreds of thousands to run one trolley
car. Yet just that slight little current from the heart is enough
to sway a gossamer strand of quartz fibre in what I may call my
'heart station' here. This current, as I have told you, passes
from each of you over a wire and vibrates a fine quartz fibre in
unison with it, one of the most delicate bits of mechanism ever
made, recording the result on a photographic film by means of a
beam of light reflected from a delicate mirror.
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