Arthur H. Bell

Aerial lines of telegraph companies were the first wires requiring lighting and foreign current protection. Long pole lines, stretching far out into the country, rarely escape injury during severe tempests, the heavy potential sweeping into the station, destroying instrument coils, and often shocking the operators and occasionally setting fire to the office. It was found that the oscillating lightning current sought first of all, a ground jump from wire to wire and instrument to instrument.

The first step toward protection, therefore, consisted of a triangular device, similar to the illustration, Fig. 1. The peg seen in the cut is metallic, and may be used to connect one side of the line directly across to the ground; in other words, is grounding that side of the line to which it is pegged. The middle wire of the trio is connected to a large plate of metal or coil of wire buried in moist earth. The two other wires are entering wires, and the lower pair the instrument wire.

Electrical Protective Devices 20

Fig. 1.

Should lightning strike along the line and enter the building, it was presumed that the foreign currents would leap across to the shield-shaped center piece and reach the ground instantly, but experience showed that high potentials often evaded the ground and continued to the instrument table.

Not, however, till the arrival of the telephone, was the serious side of lightning dealt with to any extent. The form of arrester shown in Fig. 1 was also on the first telephone instruments, being placed on top of the bell box at the binding posts. The entering wire is in reality a lead wire, which is drawn to a diameter calculated to melt and part at a certain maximum current. All wire possesses a conductivity proportionate to its area or section, and the thinner the wire the less current it will carry safely. Fuses, therefore, are designed along the lines of Fig. 2, being inserted in a cut in the line wire. It is desirable to use two fuses, one on each wire. In Fig. 2 the lead fuse wire is stuck to a mica strip, with shellac.

Electrical Protective Devices 21

Fig. 2.

It was found, however, that enormous potentials passed through small fuses without " blowing " them, because voltage is not a heating factor like amperage, and provisions had to be made to side-track the heavy voltage before it reached the instrument.

A device known as the carbon block arrestor, came next into use for this purpose. In Fig. 3 the left-hand binding post is connected to the ground wire and is in itself in contact with the left hand of the two carbon blocks. Between this block and the right-hand one which it appears to touch, is a thin wafer of mica, oiled silk or tough paper, insulating one carbon from the other by a space of .0005 in., or so. The right-hand block connects with the fuse and with the instrument. The other end of the fuse goes into the line wire. It will be seen that ordinary currents pass in through the

Electrical Protective Devices 22

Fig. 3.

fuse to the instrument, and the carbon block operates only when a potential sufficient to leap the carbon gap passes in. The further apart the blocks the greater potential needed to break down the air space. In this way the protector protects from electric light and power circuits the fuse from blowing and the block arcing. It is desirable to place wooden, asbestos lined or porcelain covers over the fuses and the carbons, to protect the premises from fire. Oftentimes the fuses come in glass tubes, and recently the fiber-covered tube has come into popularity.

In telephone practice there are hundreds of devices, designed by the engineers for certain duty. Some are similar to those just described and some are self soldering, that is after blowing, restore themselves to normal condition. The principle involved is the relation of current to time, a certain amount of excessive current passing through insulated resistance wound on a brass spool, heats a metal pin soldered with soft solder in the spool aud opens the circuit as long as the excessive current prevails.

Electrical Protective Devices 23

Fig 4.

In wireless telegraphy the aerial wire is elevated to a height of from 50 to 200 feet, and gathers at all times from the atmosphere a certain amount of high tension even on cloudless days. Such tension cannot be readily dispersed, and in most stations the chattering of the sensitive relay shows its action at times to be quite vigorous, in fact it is only recently in actual wireless practice that scientists have appreciated that there are cloudless days when, should it suddenly become dark the atmosphere would be streaked with flashes of heat (?) lightning, as on a Summer's night.

To protect wireless stations it is advisable to connect a spark gap, as in Fig. 4, one side to the aerial wire and the other to the ground whenever there is a tempest, and also throw open the knife switch used for connecting the aerial with instruments. It is most advisable to have a knife switch of the two-way type on the outside of the building, connecting the aerial to the knife blade, the station wire to one post and the ground to the other. Prompt grounding of this aerial insures absolute protection to instruments and the operator.