450 Prof. Gr. Quincke on the Measurement of Magnetic 



sulphide-of-carbon manometer observed with a cathetometer- 

 microscope *. 



When the electromagnet was excited, the difference in 

 level between the two limbs of the manometer increased by 

 an amount h ; the manometer showed an increase in pressure 

 of the air included in the flat air-bubble, which was nearly- 

 independent of the diameter of the bubble, proportional to the 

 square of the magnetic force H^ of the magnetic field, and of 

 different magnitude with different liquids. 



If we denote the specific gravity of the fluid in the mano- 

 meter by o-, the difference of magnetic pressure on the unit 

 area within the fluid and the air-bubble will be 



/^^g^Ha 2 . (2) 



Instead of sulphide of carbon, we may also use the magnetic 

 fluid itself as manometer-fluid. 



The pole-surfaces of the RuhmkorfFs electromagnet, being 

 the same distance apart as before, were placed vertically and 

 opposite to each other, and the narrow vertical limb of a 

 U-shaped glass tube was brought into the magnetic field, of 

 which the other vertical limb, of 25 millim. diameter, was placed 

 outside the magnetic field at a distance of 200 millim. The 

 U-shaped tube was so far filled with the magnetic fluid that the 

 meniscus in the narrow limb lay in the middle of the magnetic 

 field, and could be observed with a cathetometer-microscope. 

 When the electromagnet is excited, the magnetic fluid rises 

 by such an amount that the increase of hydrostatic pressure is 

 equal to the increase of pressure observed in the flat air-bubble 

 by means of the sulphide-of-carbon manometer. The increase 

 of pressure is again determined by equation 2 : if A denote 

 the increase in the difference in height of the fluid in the 

 limbs of the U-shaped tube, and a- the specific gravity of the 

 fluid, this increase in pressure takes place in a direction at 

 right angles to the lines of magnetic force. 



We find the same increase in pressure on exciting the 

 electromagnet, when we place the lines of magnetic force 

 between the poles vertical, and introduce the one leg of the 

 U-shaped tube through the central perforation of the pole, 

 so that the meniscus of the magnetic fluid is in the magnetic 

 field, and is displaced parallel to the lines of magnetic 

 force. The increase of hydrostatic pressure under the 

 influence of magnetic forces proved to be independent of 

 the diameter and length of the U-shaped tube, so long as 



* Wied. Ann. xix. p. 719, and Taf. viii. fig. 25 (1883). 



