amined by galvanometers and
decomposing apparatus introduced into the course of this wire. I will
always speak of it as the current in the cross wire or wires, so that no
mistake, as to its place or origin, may occur. In the wood-cut, Z and C
represent the zinc and copper plates of the electromotor; G and E the cups
of mercury where contact is made or broken (1052.); A and B the
terminations of D, the long wire, the helix or the electro-magnet, used to
complete the circuit; N and P are the cross wires, which can either be
brought into contact at _x_, or else have a galvanometer (1058.) or an
electrolyzing apparatus (312. 316.) interposed there.

[Illustration]

The production of the _shock_ from the current in the cross wire, whether D
was a long extended wire, or a helix, or an electro-magnet, has been
already described (1060. 1061. 1064.).

1080. The _spark_ of the cross-wire current could be produced at _x_ in the
following manner: D was made an electro-magnet; the metallic extremities at
_x_ were held close together, or rubbed lightly against each other, whilst
contact was broken at G or E. When the communication was perfect at _x_,
little or no spark appeared at G or E. When the condition of vicinity at
_x_ was favourable for the result required, a bright spark would pass there
at the moment of disjunction, _none_ occurring at G and E: this spark was
the luminous passage of the extra current through the cross-wires. When
there was no contact or passage of current at _x_, then the spark appeared
at G or E, the extra current forcing its way through the electromotor
itself. The same results were obtained by the use of the helix or the
extended wire at D in place of the electro-magnet.

1081. On introducing a fine platina wire at _x_, and employing the
electro-magnet at D, no visible effects occurred as long as contact was
continued; but on breaking contact at G or E, the fine wire was instantly
ignited and fused. A longer or thicker wire could be so adjusted at _x_ as
to show ignition, without fusion, every time the contact was broken at G
or E.

1082. It is rather difficult to obtain this effect with helices or wires,
and for very simple reasons: with the helices i, ii, or iii, there was such
retardation of the electric current, from the length of wire used, that a
full inch of platina wire one-fiftieth of an inch in diameter could be
retained ignited at the cross-wires during the _continuance of contact_, by
th