88 A. M. Mayer — Researches in Acoustics. 



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 centers 

 at H. These centers 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 apparatus. When 

 the swinging plate is vertical and the axis of the screw hori- 

 zontal 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 Ys. 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 con- 

 tact 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 -fo$ 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 centimeter 

 of the screw is heated 10°, which is a large estimate, consid- 

 ering 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 

 TeeVee °f tne length of the bar. This change in the length 

 of the screw will affect the coefficient of expansion of the bars 

 only -00000006. 



Determination of the Densities of the Bars at 4°. 



The bar, whose density was to be determined, was immersed 

 in water at 4° for a couple of hours. The bar was then sus- 

 pended by a platinum wire in water at 4° and weighed. The 

 bar was then removed from the wire and a quantity of water 

 equal in volume to the volume of the bar was added to the 

 water in the vessel, and the platinum wire, now immersed ex- 

 actly as it was when the bar was attached to it, was weighed. 

 This weight, subtracted from the previous weighing, gave the 

 weight of the bar in water. Every precaution was taken to 

 prevent, by means of screens, the action on the balance of 



