TRANSACTIONS OP SECTION G. 647 



6. A Note on 'Earth Resistance.' 

 By Professor E. W. Marchant, D.Sc.^ 



This paper described a new method of expressing the effectiveness of earth- 

 ing arrangements such as are employed for lightning-conductors, radio-tele- 

 graphic stations, and in connection with many electric-power distributing 

 systems. The method consists in determining the length of the column of earthy 

 material which, with a cross-section equal to the surface of the earth plates, 

 would have the same resistance as the actual ' earth. ' For a hemisphere 

 embedded with its flat surface level with the surface of the ground, this 

 ' equivalent length of earth ' is easily seen to be equal to the radius of the 

 hemisphere. 



Some tests were described, made by the author at Liverpool on three 

 ' earths ' of different types, one being the water-pipe system of the building. 



6. Transformer Core Loss as affected by Triple Harmonics. 

 By H. M. Lacey, B.Sc, and C. H. Stubbings, B.Sc- 



This paper described experiments conducted in the Siemens Electrical 

 Engineering Laboratory, King's College, London. The object of these experi- 

 ments was to investigate the effect of triple harmonics upon the core losses in 

 transformers. 



An alternator giving approximately a sine wave was three-phase star con- 

 nected to a three-phase choking coil. The star centres were also connected 

 through a switch. Current-measuring resistances were inserted in the neutral 

 and one of the outer leads. Voltmeters and ammeters were also inserted as 

 required. 



Two experiments were made : 



(1) With the switch in the neutral wire open. 



(2) With the switch in the neutral wire closed. 



In each case the wave forms of phase amperes, phase volts, and in the second 

 case amperes in the neutral wire, were taken by means of a Kelvin quadrant 

 electrometer and a contact-maker. It was found upon analysing the various 

 wave forms that in the first case there was a 48'9 per cent triple harmonic in 

 the volt curve but practically none in the current curve. In the second case 

 there was practically no triple harmonic in the volt curve but 48-2 per cent, in 

 the current curve ; also the wave of current in the neutral wire was very nearly 

 a sine wave of triple frequency, and having an amplitude of approximately three 

 times that of the triple harmonic in the current curve. This shows that the 

 existence of a neutral connection enables a triple harmonic to exist in the 

 current curve, and also that the suppression of this harmonic from the current 

 curve forces it into the volt curve. Using these curves, the induction curves 

 for the two cases were drawn, and from these the hysteresis loops (including 

 eddy current losses) were obtained. The areas of the two loops were measured, 

 and the ratio of the areas case 1 : case 2 was found to be 0'775, thus showing 

 that by insulating the star centres a saving of 22-5 per cent, in the core loss is 

 obtained, the voltages being equal. 



The watts supplied in the fir.st case were 24-3 per cent, less than in the 

 second case, the average watts being found from the curves. The results of 

 these experiments point to the fact that from the point of view of core losses 

 the star centres in three-phase systems should be insulated. 



7. Exposure Tests of Light Aluminium Alloys. 

 By Professor Ernest Wilson.' 



The physical properties of certain light aluminium alloys have formed the 

 subject of reports during the last fourteen years. In order to investigate the 



' See The Electrician, vol. Ixxv., p. 882. 

 ' Ibid., p. 874. ' Ibid., p. 886. 



