LIGHTNING-ARRESTERS. Devices to protect electrical apparatus from damage by lightning. In the early days of telegraphy and telephony many accidents were caused by light ning striking the wires and running into the offices, often injnring the operator and Ilestroy ing, property. In order to obviate these ties an instrument was devised and called qi:ihtning-arrester.' The original simple form is shown in Fig. I. The three plates B, C are mounted upon an insulating Hoek. For ample. the plate A is connected to the grmind.
B to the telegraph or telephone runi•nt. and C to the line wire. and to the other side of the instrument. In ordinary operation the curry its from the line cannot go from C awn ss to A with out going through the instrument. but if the line is struck by lightning the latter comes to C, and by reason of its high pressure mumps from the jagged edges of C over to A an 1 13, without going through the other instruments at all. on aceount of their high impedance (effective resistances. Thus the so-called arrester provides a cross-cut, or short-circuit, so that the lightning reaches the earth without going through the telegraph or telephone apparatus. A similar device can be used where a number of lines are to be protected. When the transmission of electric power over ex posed lines became general, much trouble was ex perienced with lightning, which would strike the line and come into the dynamos and break down their insulation at some point, passing through, and, thus establishing an arc, would 'burn out' the maehine.
Fig. 2 shows a form of arrester used with dy namos; A and B are connected to the two sides of the circuit, and C to the earth. The lightning will jump from the toothed edges of the carbon plates 1) D' over to the plates E E' and thence through C to the earth. If E and E' were both connected directly to the earth, then a lightning dis charge between DD' and EE' would es tablish a double are, thus Nhort-circuiting the dynamo through the ground. To avoid this difficulty E' is grounded through the small fuze-wire F. which will carry the lightning, but melts in two as soon as the heavy dynamo current starts. thus breaking the short circuit. There are other forms of arrester in which the lightning, jumps neross between cylin ders of so-called non-arcing metal, avoiding the use of the fuze, F, Fig. 2.
Fig. 3 shows the manner in whieh the arrester is introduced to protect the generator. Even the hest lightning-arresters do not afford com plete protection, and ninny serious accidents still occur.