448 Prof. G. Quincke on the Measurement of Magnetic 



field in question, and j? is a constant corresponding to the 

 so-called dielectric constant, and which on that account I 

 propose to call the diamagnetic constant. 



The diamagnetic constant of atmospheric air is 1, as is also 

 its dielectric constant. If a magnetic fluid is bounded by air, 

 the magnetic difference of pressure in the fluid and in the 

 air may be expressed by a hydrostatic pressure, and thus 

 ^ 1 — 1 may be measured. 



In my experiments I employed either a RuhmkorfF 's electro- 

 magnet having plane circular pole-surfaces of 140 millim. 

 diameter, at a distance from each other of 1*7 to 50 millim.; 

 or the large electromagnet of the physical collection of the 

 Royal Academy, with plane round pole-surfaces of 24 millim. 

 diameter and 3'5 millim. apart. The first of these was excited 

 by a battery of one, three, or ten Bunsen's elements arranged 

 in series ; and the second by a battery of one, two, or three 

 such elements. The magnitude of the magnetic force E^ of the 

 magnetic field between the poles was nearly constant, and was 

 measured by means of the induced currents in a small flat 

 spiral of 30 millim. or of 8 millim. diameter, and of one or 

 five coils of thin copper wire which was drawn out of or 

 pushed into the magnetic field. At the same time a large 

 induction-spiral of eleven coils and 490 millim. diameter was 

 included in the circuit leading to the galvanometer at a dis- 

 tance of 15 metres, in which, by revolution about a vertical axis, 

 the horizontal magnetic force of the earth produced induction- 

 currents. 



If we call the surfaces of the induction-spirals F and F 1? 

 2s the deflection of the galvanometer-needle when the large 

 induction-spiral is revolved through 180°, being at right 

 angles to the magnetic meridian in its initial and final posi- 

 tions, Si the deflection caused by withdrawing the small 

 flat spiral parallel to itself from the magnetic field, then the 

 magnetic force of the magnetic field is 



H '4,-? H m 



The horizontal component of the earth's magnetism was 

 measured by Dr. Walter Konig with a portable magnetic 

 theodolite of Meyerstein's, at the position of the large induc- 

 tion-spiral, and was found equal to 0*1935 C.G.S. units. 



The magnetic force of the magnet-field varied between 

 300 and 3000 C.G.S. units in the case of the RuhmkorfFs 

 electromagnet, and between 6000 and 12000 C.G.S. units in 

 the case of the Berlin electromagnet. For the sake of com- 

 parison, I may mention that in the well-known experiments 



