TRANSACTIONS OF THE SECTIONS. 29 



trical contact, and formed a bridge by which the electricity freely passed. With 

 a tension of six or seven hundred cells the electricity was found to pass through a 

 considerable interval of the dust met with in rooms, which consists chiefly of silica 

 and alumina, with more or less organic and earthy matters. 



Incandescent matter offers a very free passage to electricity ; masses of highly 

 heated blacklead-powder were found with six cells to give an average resistance of 

 four imits, and wood-charcoal powder an average resistance of five units, or about 

 ■f 3 of that opposed by an ordinary needle-telegraph coil, which may be taken at 

 300 units. These experiments went to show that an interval of dust separating 

 two conductors oppose practically a decreasing resistance to an increasing tension, 

 and led to the construction of the " lightning-bridge," which consists of two 

 pointed conductors enclosed in a chamber, and approached to within -['^ of an 

 inch from one another, and surrounded with finely divided matter consisting of 

 carbon and a non-conducting substance intimately mixed. The reason why a 

 powder consisting entirely of conducting matter cannot be safely employed is, that 

 although it opposes a practically infinite resistance to the passage of electricity of 

 the tension of ordinfiry working currents, when a high tension discharge occurs the 

 particles under the influence of the discharge generally arrange themselves so closely 

 as to make a conducting connexion between the two points of the lightning-bridge. 

 If the effect of lightning striking the wires be considered, it will be seen that the 

 electric discharge passing through the telegraph-coils is not momentary, but occu- 

 pies time. 



When an insulated telegraph-wire is struck, the effect of the electric discharge 

 is to polarize the wire throughout ; after the discharge the wire returns to its nor- 

 mal impolarized condition; but the cessation of magnetization, although rapid is 

 not instantaneous, and the eftect of the wire assuming its normal unmagnetic con- 

 dition will be to develope an electric current flowing in the same direction as the 

 electric discharge which magnetized the wire. The tension of the current developed 

 by the demagnetization will be very high at its first development, and wiU rapidly 

 afterwards fall to zero ; there is therefore, first, the main discharge of electricity 

 which passes by the shortest route, and does not wait to polarize the coil-wires, 

 but leaps across a space of air to the earth conductor as the easier course, followed 

 by a secondary current in the same direction but occupying time. The tension of 

 this secondary current, although at first very high, is not nearly so great as the 

 lightning discharge, and the greater portion, if not the whole, will pass through 

 the coils, which oppose, when time is given, a much lesser resistance than the 

 smallest possible space of air ; it would therefore seem, when telegraph circuits 

 protected by the ordinary protector are struck by lightning, it is to the secondary 

 current, and not to the main discharge the fusion must be attributed. The fact 

 that the coils of needle telegraphs are more often fused than other telegraphic ap- 

 paratus was considered to be a strong confirmation that the fusion was due to the 

 secondary current developed by the demagnetization. 



The relay coils used in other telegraph systems have soft iron cores which are 

 rendered magnetic when a ciu-rent is passing through the coils. A greater amount 

 of magnetism is developed in the cores than in needle-telegraph coils ; but a very 

 sensible time is occupied by the iron passing fi'om the normal to a magnetized con- 

 dition. The momentary resistance these cores oppose to magnetization is very 

 great ; the demagnetization of the line-wire proceeds more slowly ; the electricity 

 generated by the demagnetization being of a definite amount, the tension of the 

 secondary cui-rent is reduced proportionately. The line-wire of a telegraph circuit 

 is only a continuation of the coil-wires, and is rendered magnetic by electric polar- 

 ization in the same way as the coils, the chief diflerence being that the magnetism 

 developed in the coils is concentrated in a smaller space. When lightning strikes 

 the line-wire of a circuit in which electromagnets are used, and having an ordi- 

 nary protector, it magnetizes the^line-wire, leaps the space separating the points of 

 the protector, and does not magnetize the electromag-nets ; demagnetization of the 

 line-wire which always takes time, and which can be retarded, follows, and the re- 

 sistance whicli the soft iron cores oppose to the assumption of magnetization retards 

 the demagnetization of the line-wire, reducing the tension of the secondary cur- 

 rent. In needle-telegi'aph coUs, there being no large mass of iron to be magnetized, 



