sed the following form of
response recorder. The curves are obtained directly, by tracing the
excursion of the galvanometer spot of light on a revolving drum
(fig. 8). The drum, on which is wrapped the paper for receiving the
record, is driven by clockwork. Different speeds of revolution can be
given to it by adjustment of the clock-governor, or by changing the size
of the driving-wheel. The galvanometer spot is thrown down on the drum
by the inclined mirror M. The galvanometer deflection takes place at
right angles to the motion of the paper. A stylographic pen attached to
a carrier rests on the writing surface. The carrier slides over a rod
parallel to the drum. As has been said before, the galvanometer
deflection takes place parallel to the drum, and as long as the plant
rests unstimulated, the pen, remaining coincident with the stationary
galvanometer spot on the revolving paper, describes a straight line. If,
on stimulation, we trace the resulting excursion of the spot of light,
by moving the carrier which holds the pen, the rising portion of the
response-curve will be obtained. The galvanometer spot will then return
more or less gradually to its original position, and that part of the
curve which is traced during the process constitutes the recovery. The
ordinate in these curves represents the E.M. variation, and the abscissa
the time.

[Illustration: FIG. 8.--RESPONSE RECORDER]

We can calibrate the value of the deflection by applying a known E.M.F.
to the circuit from a compensator, and noting the deflection which
results. The speed of the clock is previously adjusted so that the
recording surface moves exactly through, say, one inch a minute. Of
course this speed can be increased to suit the particular experiment,
and in some it is as high as six inches a minute. In this simple manner
very accurate records may be made. It has the additional advantage that
one is able at once to see whether the specimen is suitable for the
purpose of investigation. A large number of records might be taken by
this means in a comparatively short time.

#Photographic recorder.#--Or the records may be made photographically. A
clockwork arrangement moves a photographic plate at a known uniform
rate, and a curve is traced on the plate by the moving spot of light.
All the records that will be given are accurate reproductions of those
obtained by one of these two methods. Photographic records are
reproduced in white against a black b