of foil, and the lower end is
washed by dipping it into strong nitric acid for a moment and thence
into water. The object of smearing the foils all over with shellac is
to prevent them being acted upon by the acid. The threads are not
very easily washed acid free, but the process may be assisted by means
of a fine camel's-hair pencil.

Some silvering solution made as described (Sec. 65) is put into a test
tube; the thread, after rinsing with distilled water, is lowered into
the solution so far as is required, and is allowed to receive a
coating of silver. It has been observed that the coating of silver
must not be too thick--not sufficiently thick to be opaque. A watch
may be kept on the process by immersing a minute strip of mica
alongside the thread.

The silvered thread is rinsed with distilled water and allowed to dry.

Meanwhile the other end of the thread may be silvered. When both ends
are silvered the process of coppering by electro deposit is commenced.
A test tube is partially filled with a ten per cent solution of
sulphate of copper, and several copper wires are dipped into it to
form an anode. The thread is lowered carefully into the solution so
as not to introduce air bubbles, and the silvered part is allowed to
project far enough above the surface of the solution to come in
contact with a fine copper wire. The circuit is closed through a
Leclanche cell and a resistance box.

It is as well to begin with a fair resistance, say 100 ohms out in the
box, and the progress of the deposit is watched by means of a
low-power microscope set up in front of the thread. If the copper
appears to come down in a granular form, the resistance is too small
and must be increased; if no headway appears to be made, the
resistance must be diminished.

As soon as a fair coat of copper has come down, i.e. when the
diameter of the thread is about doubled, the process is interrupted.
The thread is withdrawn, washed, dipped in a solution of chloride of
zinc, and carefully tinned by dragging it over a small clean drop of
solder on a soldering bit.

During this part of the process the shellac is apt to get melted if
the iron is held too close, so that it is advisable to begin by making
the thread somewhat over long. The end of the thread must only be
trimmed off at the conclusion of the operation, i.e. after the thread
is soldered up. The thread is attached to the previously tinned
supports much in the same way as