Referring to the illustration of the patent drawings of the Robjohn
machine, it will be noted that there are three sight openings in the
casing through which the registration of the numeral wheels may be
read. The numeral wheels, like those of all machines of this character,
are connected by devices of a similar nature to those in the Hill
machine for carrying the tens, one operating between the units and tens
wheel and another between the tens and hundredths wheel.
[Sidenote: _Description of Robjohn machine_]
The units wheel shown in Fig. 3 is connected by gearing to a long
pin-wheel rotor, marked E, so that any rotation of the rotor E,
will give a like rotation to the units numeral wheel to which it is
entrained by gearing.
To each of the nine digital keys, marked B, is attached an engaging and
disengaging sector gear device, which, as shown in Fig. 3, although
normally not in engagement with the rotor E, will upon depression of
its attached key, engage the rotor and turn it.
A stop device is supplied for the key action, which in turn was
supposed to stop the gear action; that seems rather doubtful. However,
an alternative device is shown in Figs. 4 and 5, which provides what
may without question be called a stop device to prevent over-rotation
of the units wheel under direct key action.
[Illustration: From Drawings of Bouchet Patent 314,561]
It will be noted that the engaging and disengaging gear device is here
shown in the form of a gear-toothed rack and that the key stem is
provided with a projecting arm ending in a downwardly projecting tooth
or detent which may engage the rotor E, and stop it at the end of the
downward key action. While the stopping of the rotor shows a control in
the Robjohn machine which takes place under direct action from the keys
to prevent overthrow of the units numeral wheel, it did not prevent the
overflow of the higher or tens wheels, if a carry should take place.
There was no provision for a control of the numeral wheels under the
action received from the carry of the tens by the transfer mechanism.
[Sidenote: _First control for a carried numeral wheel_]
The first attempt to control the carried wheel in a key-driven machine
is found in a patent issued to Bouchet in 1882 (see illustration
on opposite page); but it was a Geneva motion gearing which, as is
generally known, may act to transmit power and then act to lock the
wheel to which the power has been transmitted until it is again to be
turned through the same source. Such a geared up and locked relation
between the numeral wheels, of course, made the turning of the higher
wheel (which had been so locked) by another set of key-mechanism an
impossibility.