[PHILIP] ELECTRICAL AND MAGNETIC DISTURBANCES 215 



7th, 1922, at 6.45 p.m., during the course of a moderate thunder 

 storm, when the power was off for about five minutes, several eye 

 readings were taken. At first the galvanometer was exceptionally 

 quiet for three minutes even when its series resistance was cut out. 

 Then apparently flashes of lightning occurred in the distance, for 

 several violent deflections were noticed after a resistance of 1000 ohms 

 was introduced in the circuit. Only after 50,000 ohms resistance was 

 put in could readings be taken. One deflection of the needle showed 

 a current equivalent to 60 milliamperes through one ohm. Of course 

 such deflections were extraordinary and would be too violent and too 

 fast to be recorded on bromide paper. 



Type of Curve Caused by Earth Currents 



Most of the records were so masked by power line effects that 

 decisive evidence could only be obtained for one type of curve. This 

 was a slow steady deflection with a period ranging from 30 seconds to 

 8 minutes, generally during an interval free from deflections of other 

 types. Such deflections are only recorded on the autographic film, 

 as they take place too slowly to be detected by the eye. 



The decisive clue to the interpretation of these curves came 

 during the remarkably intense and prolonged magnetic storm of 

 May 11th to the 18th, 1921. During this period the records were 

 altogether different from those hitherto obtained; there was hardly 

 any difference between the day and the night traces, except that the 

 latter were more vigorously disturbed, especially during the height 

 of the magnetic storm. A careful comparison with the magnetic 

 records showed an almost absolute agreement both as to time and 

 value in the two curves. The type of curve obtained on this occasion 

 is shown in Fig. 2, together with the horizontal force record obtained 

 at the Magnetic Observatory, Agincourt, during the height of the 

 magnetic storm. On the upper or magnetic record is shown the trace 

 for two days. May 14 and 15, 1921, though the greater portion of 

 the record for the 14th was off the scale; below are shown the corres- 

 sponding earth current records. The time scale on the latter is much 

 more open than that on the former and corresponding points are 

 marked I and II. The similarity between these records is at once 

 apparent. The earth current record shown here is quite different 

 from the ordinary day form. Repeatedly the deflections of the 

 type observed during the magnetic storm have come into prominence 

 though, of course, with less intensity, and the agreement is always 

 noted. 



