n the brackets, the brackets being
swivelled as much as necessary to allow of this. When the pins are
all inserted the brackets are screwed up by the screws at E. The pins
are then cemented firmly to the brackets by a little plaster of Paris.
The coil carrier can now be adjusted to the galvanometer frame by
means of screws at D, which pass through wide holes in the carrier and
bold the latter in position by their heads. In the sectional plan the
parts of the galvanometer frame are shown shaded. The front of the
frame at F F is of glass, and the back of the frame is also made of
glass, though this is not shown in the section.

A represents an ebonite ring into which the wire coil is cemented by
means of paraffin. B B B B are quartz pins, with heads inside the
ebonite ring. C C C are slotted brackets adjustable to the pins and
capable of rotation by releasing the screws E E. D D are the screws
holding the coil carriage to the galvanometer framework. These screws
pass through large holes in the carriage so as to allow of some
adjustment.

Fig. 82.

Fig. 83.

Sec. 104. Glass.

When glass is properly chosen and perfectly dry it has insulating
properties possibly equal to those possessed by quartz or crystalline
sulphur. For many purposes, however, its usefulness is seriously
reduced by the persistence with which it exhibits the phenomena of
residual charge, and the difficulty that is experienced in keeping it
dry.

The insulating power of white flint glass is much in excess of that of
soft soda glass, which is a poor insulator, and of ordinary green
bottle glass. The jars of Lord Kelvin's electrometers, which insulate
very well, are made of white flint glass manufactured in Glasgow, but
it is found that occasionally a particular jar has to be rejected on
account of its refusing to insulate, and this, if I understand aright,
even when it exhibits no visible defects.

A large number of varieties of glass were tested by Dr. Hopkinson at
Messrs. Chance Bros. Works, in 1875 and 1876 (Phil. Trans, 1877),
and in 1887 (Proc. Roy. Soc. xli. 453), chiefly with a view to the
elucidation of the laws regulating the residual charge; and
incidentally some extraordinarily high insulations were noted among
the flint glasses. The glass which gave the smallest residual charge
was an "opal" glass; and flint glasses were found to insulate 105
times as well as soda lime glasses. The plates of Wimshurst machines