Dr. A. M. Mayer's Researches in Acoustics. 175 



the weight of the tube is taken off the Vs by helical springs 

 fastened to a frame above the apparatus. The tension of these 

 springs can be so regulated that the tube rests on the Vs with 

 the same pressure when it has steam passing through it and 

 when it is filled with ice. The column, A, and the Vs, V, V, 

 are insulated from the base of the apparatus by thin plates of 

 ebonite, e. Between the binding-screws, E and E f , and con- 

 nected by wires, are the voltaic cell F, the galvanometer Gr, 

 and a box of resistance-coils, R. The micrometer-screw, M', 

 with which the variations in length of a bar are measured, 

 is mounted as follows : — The screw passes through its nut in 

 a massive brass plate which rotates around nicely fitted 

 centres at H. These centres are supported by two side plates 

 not shown in the figure. A spring, K, is fastened to the 

 lower part of the swinging nut-plate and brings this plate 

 against the plate, L, firmly fastened to the base of the appa- 

 ratus. When the swinging plate is vertical and the axis of 

 the screw horizontal, the swinging plate fits accurately the 

 surface of the fixed plate, L. By turning the rod, N, the 

 swinging plate and its screw can be rotated away from the 

 bar. This arrangement allows the screw to be swung out 

 of the way while the tube, T, is being placed in the Vs. 

 Also, it prevents any strain between the micrometer-screw, M', 

 and the column, A ; which would take place if M' were fixed 

 and it should be brought in contact with a hot bar in the 

 tube, T. 



With careful manipulation, successive electric-contacts can 

 be made on a bar in the tube, T, surrounded by ice, so 

 that the variations in a series of measures will not exceed 

 Woo mm -? "with a resistance of about 200 ohms placed in 

 the circuit. 



It may be reasonably objected to this apparatus that when 

 the micrometer-screw touches the bar at 0° it is cooled and 

 shortened, and that when it touches the bar at 100°, or at 

 temperatures higher than that of the screw, the latter is 

 heated and elongated. This error, however, is quite small, 

 and may be neglected in our work. If we assume that one 

 centimetre of the screw is heated 10°, which is a large 

 estimate, considering the duration of contact of screw and 

 bar during a measure, the shortening or elongation of 1 cm. 

 of the screw by cooling or heating it 10° amounts to only 

 •0012 mm., or jqqqqq of the length of the bar. This change 

 in the length of the screw will affect the coefficient of 

 expansion of the bars only 00000006. 



