380 On the Compressibility of Solids 



bearer, and its length is as nearly as possible equal to the 

 distance which separates the inside ends of the glass terminals 

 when in position. When the pressure in the apparatus is 

 raised, both the wire and the glass tube which carries it are 

 shortened, while the steel tube which carries both of them is 

 lengthened, and when the pressure is relieved the reverse takes 

 place. The glass tube behaves exactly like the glass rod, that 

 is, it is liable to a slight motion of translation. Similarly, the 

 wire, which is carried by the glass tube, generally expands and 

 contracts under pressure at a less rate than does the glass, 

 producing again a slight apparent motion of translation. But 

 again, as in the case of the rod, the algebraic sum of the observed 

 motions is the expansion or contraction of the wire. 



There is an advantage in having a very slight leak in the 

 apparatus. The routine of an observation is then that the 

 observer in charge of the pump and the manometer gets the 

 pressure up somewhat higher than that desired; he then 

 settles himself with the relieving lever in his hand and calls 

 out as the mercury in the manometer in falling passes each 

 division. The observers at the microscopes read their micro- 

 meters at the same moment. When the pressure has fallen a 

 little below the desired pressure, the pressure is very carefully 

 relieved, and the readings of the micrometers and of the mano- 

 meter are taken at atmospheric pressure. The algebraic sum 

 of the movements of the two ends on the micrometers gives the 

 linear expansion of the body which has taken place, and the 

 difference of the two readings of the manometer gives, when 

 interpreted by the help of Piezometer C. No. I, the difference 

 of pressure which has caused the expansion. The micrometer 

 measurements are then reduced separately to their absolute 

 values in terms of the inch. The algebraic sum then gives the 

 linear expansion in terms of the inch. It is then divided by 

 the length of the rod or wire in inches and by the pressure in 

 atmospheres; the resulting quotient is the linear compressi- 

 bility of the metal or other substance. Multiplying this by 

 three, we obtain the cubic compressibility of the substance, 

 if truly isotropic. 



It will be evident that, to work with this instrument, three 



