were
actuated through polarized relays, as occasion required, one polarity
energizing the electromagnet controlling the type-wheels, and the
opposite polarity energizing the electromagnet controlling the printing.
Later on, however, he changed over to a two-wire circuit, such as
shown in Fig. 2 of this article in connection with the universal stock
printer. In the earliest days of the stock printer, Edison realized
the vital commercial importance of having all instruments recording
precisely alike at the same moment, and it was he who first devised (in
1869) the "unison stop," by means of which all connected instruments
could at any moment be brought to zero from the central transmitting
station, and thus be made to work in correspondence with the central
instrument and with one another. He also originated the idea of using
only one inking-pad and shifting it from side to side to ink the
type-wheels. It was also in Edison's stock printer that the principle of
shifting type-wheels was first employed. Hence it will be seen that,
as in many other arts, he made a lasting impression in this one by the
intrinsic merits of the improvements resulting from his work therein.

We shall not attempt to digest the forty-six patents above named, nor to
follow Edison through the progressive steps which led to the completion
of his universal printer, but shall simply present a sketch of the
instrument itself, and follow with a very brief and general explanation
of its theory. The Edison universal printer, as it virtually appears
in practice, is illustrated in Fig. 1 below, from which it will be seen
that the most prominent parts are the two type-wheels, the inking-pad,
and the paper tape feeding from the reel, all appropriately placed in a
substantial framework.

The electromagnets and other actuating mechanism cannot be seen plainly
in this figure, but are produced diagrammatically in Fig. 2, and
somewhat enlarged for convenience of explanation.

It will be seen that there are two electromagnets, one of which, TM, is
known as the "type-magnet," and the other, PM, as the "press-magnet,"
the former having to do with the operation of the type-wheels, and the
latter with the pressing of the paper tape against them. As will be seen
from the diagram, the armature, A, of the type-magnet has an extension
arm, on the end of which is an escapement engaging with a toothed wheel
placed at the extremity of the shaft carrying the type-wheels. This