54 SEC. 3. MEASUREMENT. 



vertical section, but not in a front view, as represented in the sketch, but 

 they have been marked in the drawing for the purpose of rendering the latter 

 more intelligible. 



The measurement of the length of the lever at the different cavities is 

 accomplished by means of a spherometer. The lower bar b is arranged in 

 quite an analogous manner as the upper bar, with this difference, that this 

 one presses against the lower extreme surface of the staff to be measured, i.e., 

 in an upward direction, whilst the one just described presses upon the upper 

 end surface, consequently in a downward direction. 



In order to meet the requirement that the levers should only press against 

 or touch the end surfaces but slightly, in the manner indicated in the drawing, 

 small balancing blocks / can be attached at any. part to the levers. 



JFor the purpose of reading two telescopes with vertical scales are required, 

 which must be placed in juxtaposition, that is to say, by the side of each 

 other. Presuming the staff to move upwards and downwards without varying 

 its length, the difference in the reading in the upper and in the lower mirror 

 will naturally be of the same value in every position of the staff. 



A glass tube has been chosen to serve as a column a, because glass possesses 

 a very small coefficient of expansion. Moreover, in using the instrument, 

 the tube-must be filled with water and two thermometers placed in it, by means 

 of which any change in the temperature that may take place during the 

 process of measuring can be accurately determined. 



A similar, although less perfect, apparatus has been employed by the 

 exhibitor in two experiments already, namely, " as regards the variations in 

 " the elasticity and the length of a wire under the influence of a galvanic 

 " current." See Transactions of the Academy of Sciences at Vienna, 

 Vol. LXVIL, Part II., April 1873; and "respecting the moderation of 

 " the torsion oscillations of wires." See Transaction of the Academy of 

 Sciences at Vienna, Vol. LXIX., Part II., March 1874. Extracts of both 

 treatises have also been published by Pogg. Ann. 



The apparatus can be employed in measuring the coefficients of expansion, 

 coefficients of elasticity, the after effects of elasticity, the expansion produced 

 by magnetism, &c., and will secure in every case an accuracy not hitherto 

 achieved, not only by reason of the correctness of the readings, but also 

 on account of the correction of temperature rendered possible through the 

 employment of the glass column. 



The measurement is likewise very easy of accomplishment, since a manipu- 

 lation such as is the case with ordinary cathetometers is not required, as the 

 variations taking place in the wire can be perceived through the telescope 

 directly magnified and projected on the scale. 



In most cases it will only be necessary in making such experiments to know 

 exactly the absolute length of the body to be measured in equal per cents, as 

 well as the elongation, for measuring which a good scale, or a very simple 

 cathetometer, is all that will be required. 



24 la. Original Cathetometer by Dulong. 



Polytechnic School, Paris. 



241 b. Cathetometer, with two Levelling Micrometer 

 Telescopes. 



Physical Science Cabinet of the Imperial Academy of 

 Sciences at St. Petersburg. 



241c. Drawing of a small Cathetometer, used by Prof. 

 Mendeleeff in his investigations on the tension of gases. 



