Forces by means of Hydrostatic Pressure. 451 



the magnetic force was constant at all points of the fluid 

 meniscus. 



If a flat bubble of air in a magnetic fluid is placed below 

 the upper horizontal pole-surface, so that it does not touch 

 the lower pole-surface, and the bubble of air observed with a 

 horizontal cathetometer-microscope, the form of the air- 

 bubble does not change when the electromagnet is excited. 



The capillarity constant of the magnetic fluid at its junction 

 with air is therefore not perceptibly altered by the magnetic 

 forces. The increase of the pressure on the air-bubble within 

 the magnetic fluid, produced by the magnetic forces, is 

 the same parallel and at right angles to the lines of magnetic 

 force. 



A similar air-bubble in an insulating fluid between the 

 horizontal plates of a condenser, changes its form very con- 

 siderably when the condenser is charged, becoming longer 

 when parallel to the lines of electric force, and narrower at 

 right angles to the lines of electric force. In order to observe 

 this difference between the stresses produced by electricity 

 and by magnetism also in the stronger magnetic field of the 

 Berlin electromagnet, the great weight of which (eight 

 hundredweight) permitted no alteration in the position of 

 the horizontal lines of magnetic force, a different method was 

 adopted. A thin solid flint-glass thread was bent at right 

 angles and placed with the vertical portion cut off at right 

 angles between the vertical pole-faces of the electromagnet, 

 and a drop of fluid was hung at the lower end of the thread. 

 The hanging drop, illuminated from above, was observed from 

 beneath by means of a right-angled prism and the catheto- 

 meter-microscope. When the electromagnet was excited by 

 means of three Bunsen/s elements, the horizontal diameter of 

 the drop remained unaltered parallel and at right angles to 

 the lines of magnetic force, with the magnetic fluids 



Manganese sulphate of sp. gr. 1*4688 



Manganese chloride 1*3695 



Ferric chloride 1*5083 



Cobalt sulphate 1*2584 



and with the diamagnetic fluid water. 



Further, a drop of iron amalgam, hanging on to an 

 amalgamated copper-wire, retained the same diameter parallel 

 and at right angles to the lines of magnetic force. If any 

 difference exists, it cannot amount to so much as J per cent. 



There is, however, a considerable change of form in the 

 hanging drop if the magnetic field does not possess equal 

 magnetic force at all points. The fact that insulating fluids 



