In general it may be said that the packing difficulty has been almost entirely eliminated, not by the employment of remedial devices, such as those often proposed for stirring up the carbon, but by preventing the trouble by the design and manufacture of the instruments in such forms that they will not be subject to the evil.

Carrying Capacity.  Obviously, the power of a transmitter is dependent on the amount of current that it may carry, as well as on the amount of variation that it may make in the resistance of the path through it.  Granular carbon transmitters are capable of carrying much heavier current than the old Blake or other single or multiple electrode types.  If forced to carry too much current, however, the same frying or sizzling sound is noticeable as in the earlier types.  This is due to the heating of the electrodes and to small arcs that occur between the electrodes and the granules.

One way to increase the current-carrying capacity of a transmitter is to increase the area of its electrodes, but a limit is soon reached in this direction owing to the increased inertia of the moving electrode, which necessarily comes with its larger size.

The carrying capacity of transmitters may also be increased by providing special means for carrying away the heat generated in the variable-resistance medium.  Several schemes have been proposed for this.  One is to employ unusually heavy metal for the electrode chamber, and this practice is best exemplified in the White solid-back instrument.  It has also been proposed by others to water-jacket the electrode chamber, and also to keep it cool by placing it in close proximity to the relatively cool joints of a thermopile.  Neither of these two latter schemes seems to be warranted in ordinary commercial practice.

Sensitiveness.  In all the transmitters so far discussed damping springs of one form or another have been employed to reduce the sensitiveness of the instrument.  For ordinary commercial use too great a degree of sensitiveness is a fault, as has already been pointed out.  There are, however, certain adaptations of the telephone transmitter which make a maximum degree of sensitiveness desirable.  One of these adaptations is found in the telephone equipments for assisting partially deaf people to hear.  In these the transmitter is carried on some portion of the body of the deaf person, the receiver is strapped or otherwise held at his ear, and a battery for furnishing the current is carried in his pocket.  It is not feasible, for this sort of use, that the sound which this transmitter is to reproduce shall always occur immediately in front of the transmitter.  It more often occurs at a distance of several feet.  For this reason the transmitter is made as sensitive as possible, and yet is so constructed that it will not be caused to produce too loud or unduly harsh sounds in response to a loud sound taking place immediately in front of it.  Another adaptation of such highly sensitive transmitters is found in the special intercommunicating telephone systems for use between the various departments or desks in business offices.  In these it is desirable that the transmitter shall be able to respond adequately to sounds occurring anywhere in a small-sized room, for instance.