A pair of Cook tubular fuses in an individual mounting is shown in Fig. 221.  Fuses of this type are not open at one end like a gun, but opportunity for the heated gases to escape exists at the caps.  The tubes are made of wood, of lava, or of porcelain.

Fig. 222 is another tubular fuse, the section showing the arrangement of asbestos lining which serves the two purposes of muffling the sound of the discharge and absorbing and cooling the resulting gases.

[Illustration:  Fig. 221.  Pair of Wooden Tube Fuses]

Air-Gap vs.  Fuse Arresters. It is hoped that the student grasps clearly the distinction between the purposes of air-gap and fuse arresters.  The air-gap arrester acts in response to high voltages, either of lightning or of high-tension power circuits.  The fuse acts in response to a certain current value flowing through it and this minimum current in well-designed protectors for telephone lines is not very small.  Usually it is several times larger than the maximum current apparatus in the line can safely carry.  Fuses can be made so delicate as to operate on the very smallest current which could injure apparatus and the earlier protective systems depended on such an arrangement.  The difficulty with such delicate fuses is that they are not robust enough to be reliable, and, worse still, they change their carrying capacity with age and are not uniform in operation in different surroundings and at different temperatures.  They are also sensitive to lightning discharges, which they have no power to stop or to divert.

Protection Against Sneak Currents.  For these reasons, a system containing fuses and air-gap arresters only, does not protect against abnormal currents which are continuous and small, though large enough to injure apparatus because continuous.  These currents have come to be known as sneak currents, a term more descriptive than elegant.  Sneak currents though small, may, when allowed to flow for a long time through the winding of an electromagnet for instance, develop enough heat to char or injure the insulation.  They are the more dangerous because insidious.

[Illustration:  Fig. 222.  Tubular Fuse with Asbestos Filling]

Sneak-Current Arresters. As typical of sneak-current arresters, Fig. 223 shows the principle, though not the exact form, of an arrester once widely used in telephone and signal lines.  The normal path from the line to the apparatus is through a small coil of fine wire imbedded in sealing wax.  A spring forms a branch path from the line and has a tension which would cause it to bear against the ground contact if it were allowed to do so.  It is prevented from touching that contact normally by a string between itself and a rigid support.  The string is cut at its middle and the knotted ends as thus cut are imbedded in the sealing wax which contains the coil.

[Illustration:  Fig. 223.  Principle of Sneak-Current Arrester]