758 REPORT— 1898. 



Both sliould be constructed of nnn-uiagnetic substances ; this is a necessity 

 both for metallic connecting pieces and for the stone or other material of which 

 the building is constructed. 



Another condition is that temperature variations should be reduced as much as 

 possible, especially in the vaiiation observatory. 



All materials employed for the construction of the magnetic observatories at 

 De Bilt, near Utrecht, in the Netherlands, have been subjected to a most rigorous 

 examination as to their magnetic condition before they were accepted for the 

 buildings. 



The observatory for absolute measurements consists of two rooms, one built of 

 stone and wood, the walls being double, and the interstices filled with sawdust. 

 It contains six pillars for making absolute measurements, at a distance from each 

 other of four metres, to enable comparisons of instruments and methods to be made. 

 It has no windows except in the ceiling, one above each of the pillars. The other 

 room, of smaller dimensions, has large windows on three sides for making astro- 

 nomical observations. 



The observatory for variation observations consists of two rooms, wholly sepa- 

 rated from each other and surrounded by a corridor. The whole is enclosed in a 

 building which has double walls, 1 60 metre thick, the interstices of which are 

 filled with loose dry peat moss, &c. Daylight is not admitted into this observatory. 

 Diurnal temperature variations are by these means wholly excluded. 



TUESDAY, SEPTEMBER 13. 



A joint meeting with Sections A and G was held for the purpose 

 of discussing the Magnetic and Electrolytic Effects of Electric Railways, 

 at which the following Papers were read : — 



1. On the Disturbance of Magnetic Observatories by Electric Railways.^ 

 By W. VON Bezold, Director of the J'otsdam Observatory. 



The demand of the authorities of the Potsdam Observatory that no electric 

 railway with an earth return should be allowed to approach them within 15 kilo- 

 metres having been characterised as exorbitant, the author states in this paper 

 some of the reasons which have led to its being made. It was felt that a distance 

 sufficiently great to prevent any possibility of disturbance should be temporarily 

 fixed till experiments had been made to find to what distance from such railways 

 the earth currents were still perceptible, and how these distances W'ere afiected by 

 the condition of the soil. In the meantime it is known that a variable current 

 like that of an electric tramway has been detected telephonically by Strecker at a 

 distance of 17 kilometres; that at Greenwich, G'84 kilometres from the line of the 

 South London Electric Railway, the disturbances of the horizontal/orce due to the 

 railway vary between -00004 and -00007 C.G.S. unit, and of the vertical force 

 between -00604 and -00009 C.G.S. unit; while at Washington, 420 metres from 

 an electric railway, the disturbances of the horizontal amount to -00010, and of 

 the vertical force to -0030 C.G.S. unit; and at Toronto, 120 metres from a line, to 

 •0012 and •0037 C.G.S. unit respectively. The effect of such disturbances on 

 magnetic measurements, which are at present made almost universally to •OOOOI 

 C.G.S. unit, will be readily understood, and when it is remembered what important 

 results have followed a further increa.se of sensitiveness of the instruments at 

 Potsdam, the limit of 15 kilometres seems small enough. While the author is 

 anxious that no unnecessary hindrance should be placed in the way of tramway 

 engineers, and assures them that the limit will be lowered as soon &s experiment 

 has shown them that it can be done with safety, he points out that they have a 

 simple remedy in their own hands — i.e. to provide an insulated return. 



' For the complete paper see Elektroteclinisclie ZeitscJtrift, part 24, 1898. 



