286 M. G. Lippmann on the Connexion between 



Also, instead of applying compensating pressure, successful 

 attempts were made to measure micrometrically the displacement 

 of the meniscus which took place each time. The drawn-out 

 tube is then fused-to at the outer end, and contains a quantity 

 of compressed air, which produces the necessary constant pres- 

 sure. The electrometer is then a few centimetres in length ; it 

 must, however, be empirically graduated. It is best when the 

 electrometric glass point presses like a spring against the side 

 of the tube which contains the dilute acid, so that under all 

 magnifying-powers it may be conveniently observed. 



Electrocapillary Engine, fig. 5. 



Just as electromagnetic motors have been constructed which, 

 depending on the principles of electromagnetism, derive their 

 work from the battery and are driven by magnetic forces, so 

 also an electrocapillary motor has been constructed which is 

 driven by capillary forces, and transforms electrical into mecha- 

 nical work. Such a motor, which is now in the Physical Insti- 

 tute at Heidelberg, consists of a glass trough, KK, filled with 

 dilute sulphuric acid (15 per cent, by volume of acid), and con- 

 taining two glass vessels, G, G y , which are partly filled with mer- 

 cury. These masses of mercury, G, G, can severally be connected 

 with the poles of a DanielFs battery, D, by platinum wires, the 

 upper parts of which are kept from contact with the mercury by 

 means of fine glass tubes. In the circuit there is a commutator, 

 W, by which these connexions may be reversed, so that each 

 mass of mercury may be successively polarized with hydrogen. 

 On each mass of mercury floats a bundle of glass tubes, B, B, 

 which are about 2 millims. in diameter and are open both at 

 the top and at the bottom. About 300 of these tubes form a 

 vertical bundle 60 millims. in height and of the same diameter, 

 and are held together by platinum wire. In the axis of the 

 bundle a glass rod is fastened, which plays the part of the stem 

 of a piston. The lower half of the bundle floats in the mercury, 

 while the upper is immersed in the acid and is quite filled with 

 it. The bundle floats freely upon the mercury and remains 

 vertical because the glass rod is clamped in a metal U-shaped 

 rod, the lower ends of which are not free but fixed to the lower 

 surface of a double horizontal lever, which is movable about a 

 fixed horizontal axis, and which can oscillate like a scale-beam. 

 Thus the bundles may be said to hang to it upwards as the scale 

 pans downwards, and keep each other in equilibrium. By 

 means of the vertical stirrup V, the rod s, and the bent lever z, 

 the oscillating motion of the lever may be converted into a rota- 

 ting one of the flywheel R ; on the axle of the flywheel is another 

 bent lever z, which moves the commutator. The whole has a cer- 



