The inventor of the new machine conceived the idea that, if the
time-stamp were provided with guides or gauges so that the card could
be placed both times in the same position, and the two records of the
time stamp thus be superimposed concentrically (as illustrated in Fig.
3), the value of the period would be represented by the arc marked off
by the initial and final imprints of the minute hand, so that, instead
of subtracting one record from another, he had only to find the value
of the arc marked off by counting the corresponding number of minutes
along the dial.
The inventor had thus gotten rid of the subtraction, but there were
several desirable qualities not yet obtained. First, he could not tell
from the record alone, whether it was the longer or the shorter arc
marked off that was the measure of the period. For instance, he could
not tell whether the period was 7 or 53 minutes. This was because the
two hand or pointer imprints were exactly alike except in position.
So he conceived the idea of making the pointer imprints different in
appearance, by providing the pointer die with a mark in line with the
pointer, as illustrated in Fig. 4.
The mark and pointer revolve together and either the dies or the platen
are so arranged that the mark can be printed without the pointer at the
initial imprint and the pointer at the final imprint as in Fig. 5, the
mark being printed or not at the final imprint, as desired. This could
be done either by allowing the pointer die or the corresponding portion
of the platen to remain retracted from the paper during the first
printing.
[Illustration:
9:23 FIG. 2. 10:15
Initial Time Stamp Record. Final Time Stamp Record.
Elapsed Time: 10:15-9:23 = 52 minutes.
To read this record, hours and minutes must be subtracted from hours
and minutes, an operation liable to much error.]
It could thus be told with certainty from the record alone whether
the longer or the shorter arc is the measure of the period, because
the beginning of the arc is that indicated by the imprint of the mark
without the pointer.
There was still something to be desired. The counting of the minutes
along the measuring arc was a waste of time, if the value of the arc
could in some way be directly indicated. If the hand were set back
to 12 o'clock for the initial imprint, the final imprint would show
the hand pointing directly at the minute whose number on the dial is
the value of the period, and it would not even be necessary to count.
But the setting of the hand back to zero would prevent its making
the final imprint of any previously begun record, so that the machine
could only be used for one record at a time. It was desirable to have
a machine that would record any number of overlapping intervals at
the same time, so that one machine would record the intervals of all
the telephone conversations under the control of a single operator,
or rather of two operators, because both of them could reach the
same machine. So it wouldn't do to set the hand back to zero, as the
hand must rotate constantly and uniformly. Then why not set the zero
up to the hand at each initial imprint? This meant making the dial
rotatable, as well as the hand. It gave an initial record like that
shown in Fig. 6.
[Illustration: FIG. 3.
Subtraction eliminated but counting still required and uncertainty
whether elapsed period is 7 or 53 minutes.]
[Illustration: FIG. 4.