62 SEC. 3. MEASUREMENT. 



The arrangement of the apparatus is as follows : 



To a solid brass pedestal, which rests on three adjusting screws, a strong 

 glass tube a 1^ meter long is cemented, to which two brass bars bb are 

 clamped. Each of these brass bars consists of two parts, of which one 

 moves or slides in the other in such a manner that it can be either lengthened 

 or shortened. The set screw c serves for fixing the chosen length. By means 

 of the joint d a horizontal rotatory or veering movement of the fore-part of the 

 bar can be effected. Close to its free end there is on each side a steel point 

 the two forming together an axis which is held by a bow ff carrying two 

 cups, in such a manner that the bow can be easily but surely turned round 

 this axis. 



The lever gg which must be firmly connected with the bow, has longitu- 

 dinally a slit, or slide, with two sliding pegs placed in a level position with 

 the axis and fastened to the bow, along which the lever can be moved, so that 

 at whatever distance from the axis the extreme end of the lever may be fixed 

 it must always turn with the bow around the axis. 



In order not to be obliged to take the measurement of the length of the 

 lever afresh at each experiment, it is provided at its upper surface with 

 conical-shaped cavities in which the screws hh catch. These conical-shaped 

 cavities would be, properly speaking, visible only in a drawing of a 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 description 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 

 a manner quite similar to the upper bar. 



In order to meet the requirement that the levers should but lightly press 

 against or touch the end surfaces in the manner indicated in the drawing, small 

 balancing blocks II can be attached at any point to the levers. 



For 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 in 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 

 attained, 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 



