656 KEPORT — 1892. 



phone circuits, and it accounts partially for tbe great difference in my ke law 

 between the observed effects of aerial and those of subterranean, or submarine, 

 conductors. The former are always much superior to the latter. This would be 

 due to the absence of polarisation in air as well as to other electromagnetic and 

 electrostatic causes. No work would be done on the molecules of air, no energy 

 would therefore be absorbed, and no retardation of currents from this cause would 

 ensue. An air dielectric is clearly the * Holy Grail ' of the electrician. 



10. Earth-currents. By W. H. Preece, F.B.S. 



The year 1892 has been remarkable in tbe number and severity of those electric 

 storms which interfere so seriously with telegraphic communication. Unfortu- 

 nately, observations made upon them on working circuits are rarely reliable. Their 

 arrival and departure are sudden ; they come when they are least expected, and 

 they occur in busy hours, when the wires are fully occupied with more remunera- 

 tive currents, and when the staff is fully engaged. Their presence, duration, and 

 relative intensity are so exactly coincident with magnetic disturbances that the 

 recorded observations in magnetic observatories of needle variations may be taken 

 to indicate tbe same elements of earth-currents also, but they do not give their 

 direction or their absolute intensity. Their general direction is determined by 

 the simultaneous observation of many circuits running in many directions, and 

 noting the lines of maximum and those of no disturbance. This gives the position 

 of the equipotential planes on the surface of the earth, and the intensity of the 

 currents gives the difference of potential between these planes. 



I have not succeeded in obtaining definite facts to prove that these planes are 

 perpendicular to the line joining the centre of the earth and the centre of the sun, 

 but there is little doubt that it is so, for telegraph circuits running in this direction 

 experience the maximum disturbances, while those running at right angles on the 

 equipotential planes suffer no disturbance. Those that experience the maximum 

 disturbance in the forenoon are not affected in the evening, and vice versd. 



The storms commenced on January 4, and they have been observed at frequent 

 intervals since. 



The great storms occurred on February 13, March 12, April 24-27, May 18, 

 July 16. 



The intensity of the maximum currents observed in England frequently ex- 

 ceeded '045 ampere (or 45 milliamperes), which much exceeds in strength the 

 working currents. Hence ordinary telegraph working was paralysed, and we had 

 to resort to metallic circuits and to the use of condensers to maintain telegraphic 

 communication. 



The effect of such currents on our railways might be positively dangerous, for 

 they ring bells and reverse the block signals. 



•045 ampere means '54 volt per mile, so that the equipotential planes differing 

 by 1 volt would have been placed about two miles apart. 



These earth-currents, though variable, are always continuous, and give no in- 

 dication of alternations. 



The simultaneous appearance of aurorse shows that disturbances on the sun's 

 photosphere, as indicated by sun-spots, produce electrostatic and electromagnetic 

 effects on the earth's surface of enormous magnitude. 



The appearance of earth-currents is invariably simultaneous over the whole 

 globe. 



Observers abroad would benefit science if they would record the maximum 

 strength, in milliamperes, of the currents received, the geographical direction of 

 the terminal earths of the circuits affected and unaffected, the resistance in ohms 

 of the circuits on which the currents are measured, and if they were to give the 

 times of observation in Greenwich and not in local time. 



