52 SEC. 3. MEASUREMENT. 



293. Collection of Timber Callipers for the use of 



foresters. C. Staudinger and Co. (F. W. von Gehren), Gicssen. 



A collection of tree-clamps (" Baumklieppen "), mostly in use for the pur- 

 pose of comparison with those of Staudinger's construction, by many autho- 

 rities recognised as the best. A list of the names is added to the collection. 



298. Calliper Compasses, with plane contact lever. 



Physical Institute of the University of Kiel (Prof. Dr. 

 G. Karsteri). 



This apparatus, which is in the possession of the Physical Science Institute 

 of the University of Kiel, was constructed in 1832 by Repsold, and made use 

 of by Schumacher for comparing the platinum kilogramme of the archives 

 with the Danish. 



(See Schum. Astronom. Jahrbuch, 1836, p. 243.) 



A description of the instrument will be found in G. Karsten, " Vom Maasse 

 und vom Messen," vol. I. of the "Encyclopaedia der Physik," p. 506 and 

 following. 



308. Apparatus for measuring the Thickness of Thin 



Plates. * R. Fuess^ Berlin. 



The instrument serves for demonstrating the bending of strong masses of 

 metal by very little pressure and the expansion by heat ; also for determining 

 the expansion coefficients of bars of 1 cm. length. 



D. CATHETOMETERS. 



241. Differential Cathetometer, an apparatus designed 

 for measuring variations in the length of solid bodies, particularly 

 of rods and wires. Dr. Heinrich Streintz, University of Gratz. 



The principle on which this apparatus is based is, reading by reflection from 

 two mirrors. Two levers, having small mirrors s attached to them perpendi- 

 cular to their axis, are turned by the flat ends of the bar to be measured as 

 indicated in the drawing. If a telescope and a scale are placed, at some 

 distance, in such a position that the image of the scale reflected by the mirror 

 is visible through the telescope, each variation in the position of the point at 

 the end of the lever will be magnified to a degree indicated by the quotient, 

 the numerator of which represents the double distance of the mirror from the 

 scale, and the denominator that of the point from the axis of rotation. 



As the latter distance can be diminished to one centimeter in the apparatus, 

 and as, moreover, the telescope with the scale can be placed at any distance 

 within which distinct images will be seen, say five metres, a shifting of the 

 reflected image by one millimeter will be equal to displacement of the end- 

 surface of the bar to be measured by * 00 1 millimeter. As, however, the 

 tenths of the millimeter can be still pretty accurately determined, the reading 

 will be correct as far as the 10,000th part. 



There is no doubt that the correctness of the reading with this apparatus 

 can be carried still further, if mirrors of superior quality and powerful 

 telescopes are employed. 



In measuring the variations in the length of wire a flat surface must be 

 given to that part on which the wire is to be suspended, as well as to the part 

 on which the weights are placed, and to which as flat surfaces the levers are to 

 be applied. 



