pable of giving good results when used in connection with the proper
form of granular carbon, but, on the whole, the tendency has been to
abandon all forms of electrode material except carbon, and its use is
now well nigh universal.

_Preparation of Carbon_. The granular carbon is prepared from
carefully selected anthracite coal, which is specially treated by
roasting or "re-carbonizing" and is then crushed to approximately the
proper fineness. The crushed carbon is then screened with extreme care
to eliminate all dust and to retain only granules of uniform size.

Packing. In the earlier forms of granular-carbon transmitters a
great deal of trouble was experienced due to the so-called packing of
the instrument. This, as the term indicates, was a trouble due to the
tendency of the carbon granules to settle into a compact mass and thus
not respond to the variable pressure. This was sometimes due to the
presence of moisture in the electrode chamber; sometimes to the
employment of granules of varying sizes, so that they would finally
arrange themselves under the vibration of the diaphragm into a fairly
compact mass; or sometimes, and more frequently, to the granules in
some way wedging the two electrodes apart and holding them at a
greater distance from each other than their normal distance. The
trouble due to moisture has been entirely eliminated by so sealing the
granule chambers as to prevent the entrance of moisture. The trouble
due to the lack of uniformity in size of the granules has been
entirely eliminated by making them all of one size and by making them
of sufficient hardness so that they would not break up into granules
of smaller size. The trouble due to the settling of the granules and
wedging the electrodes apart has been practically eliminated in
well-designed instruments, by great mechanical nicety in manufacture.

Almost any transmitter may be packed by drawing the diaphragm forward
so as to widely separate the electrodes. This allows the granules to
settle to a lower level than they normally occupy and when the
diaphragm is released and attempts to resume its normal position it is
prevented from doing so by the mass of granules between. Transmitters
of the early types could be packed by placing the lips against the
mouthpiece and drawing in the breath. The slots now provided at the
base of standard mouthpieces effectually prevent this.

In general it may be said that the packing difficulty has been almo