75Ö 



which has been prepared and measured with tlie utmost care, the 

 two apertures in the steel piece A and the two sections of the 

 cjiindre B are ground in such a way that the difference between 

 the larger and the smaller section has a definite size, e.g. 1 cm^ 

 If we now suppose a pressure e.g. of 800 kg. per cm'' to prevail 

 in the space C, and no friction to be present, the piston B would 

 be forced upwards with a force of 800 kg. If on the plate D, 

 which is connected with B by means of the socket joint E we 

 put so many weights that they together with B, D, E and the 

 joining-rod E weigh exactly 800 kg., the whole apparatus is exactly 

 in equilibrium. If the total weight amounts to 801 kg. — we 

 still suppose absence of friction — the piston descends till the 

 liquid in C and the space in conneclion with it is compressed so 

 much that there [)re\'ails a pressure of 801 kg. per cm'' in C. 

 If the total weight amounts to 799 kg., the piston rises till the 

 pressure in C has fallen to 799 kg. per cm'. On account of the 

 strong friction of the })iston very tightly fitting in the cylindre 

 nothing, however, is to be observed of these movements. In fact 

 the plate D can easily be loaded with 10 or 20 kg. too much or 

 too little without any movement being perceived on a manometer 

 connected with C'. If, howe\er, the piston B with the plate 1) and 

 all the weights lying on it are made to rotate round their axis, it 

 appears that this rotation has practically annihilated all the friction. 

 It will appear from the description of our experiments that the 

 remaining friction will lie far below 10 gr. at low pressures, and 

 that it can certainly not be so much as 50 gr. for pressures of 

 2500 kg. 



We cannot account for this most remarkable property, though it 

 is of course clear that the fact that C is filled with machine oil, 

 and that tliis oil penetrates between piston and cylindre wall plays 

 an important part in this. It is known that also in Amagat's mano- 

 meter the great decrease of friction when the piston moves w^ith 

 respect to the cylindre wall is utilized. But in Amagat's mano- 

 meter ^) the piston must be moved to the left and the right by hand, 

 also during the measurement. The mode of construction of Schaffer 

 and Budenberg's pressure balance evades this by making the whole 

 mass of the weights, for the large model up to 1250 kg., for the 

 small one up to 250 kg. rotate with the piston B. After these 



1) This manometer is generally called after Desqoffe ; according to Amagat, 

 however, the first idea came from Gally — Gazalat. And we owe to Amagat the 

 great improvement, which rendered the instrument for the first time adapted for 

 really accurate measurements, nl. the free movability of the pistons. 



