SCIENCE- G OS SIP. 



is completely surrounded by a water-jacket, by 

 means of which its temperature can be varied as 

 desired. A tube containing mercury is connected 

 to the above tube by means of an india-rubber 

 joint, and is supported on a narrow board hinged 

 to the stand. The pressure upon the gas is regu- 

 lated by varying the inclination of the mercury 

 tube, which is done by means of an adjustable 

 wedge. A convenient mark of reference on the 

 tube containing the gas is afforded by the top sur- 

 face of the cork fixing the water-jacket, one level 

 of the mercury being always brought to that 

 position in an experiment, in order to keep the gas 



at constant volume. 

 The different tem- 

 peratures to which 

 the gas is brought 

 are ascertained by 

 means of a thermo- 

 meter placed in the 

 water-jacket, while 

 the pressures upon 

 the gas due to the 

 mercury are mea- 

 sured by the vertical 

 differences in height 

 of the two ends of the 

 column. To obtain 

 the total pressure 

 on the gas the atmo- 

 spheric pressure 

 must be added in 

 each case. A most 

 useful adjunct to 

 both this apparatus 

 and that shown in 

 fig. 1 is a point cathetometer for measuring differ- 

 ences in height. This instrument has already been 

 described and illustrated in the Physics column of 

 Science-Gossip for Februarv 1900. 



-Simultaneous 

 Apparatus. 



of >ilk thread, having tied to its other end a mag- 

 netised steel needle. Mercury is pound into the 

 funnel until the free pole of the needle jusl tioats 

 above the surface. To the baseof the instrument is 

 fixed a vertical brass rod bent at right angles, and 

 having suspended from the end of its horizontal 

 limb a vertical wire almost overhanging the free 

 pole of the needle, and making contact with the 

 mercury. Rotation of the two wires round one 

 another is set up when a battery current Ls sent in 

 at one terminal upon the base, through the sus- 

 pended wire, then through the mercury, and out at 

 the second terminal. 



Absolute Expansion <>k Solids. 



Until comparatively recently there was a dearth 

 of simple pieces of apparatus for the determination 

 in absolute measure of the expansion of solids. 1 

 have given a full description in SCIENCE-GOSSIP 

 (December, 1898, pp. 197-8) of a most useful and 

 efficient instrument for this purpose, designed by 

 Mr. F. C. Weedon. Simpler arrangements, but 

 upon the same principle, have been devised, some 

 depending upon the use of micrometer screw 

 gauges for the determination of the differences in 

 length, others upon the readings of spherometers. 

 The form illustrated in fig. 5 utilises a wedge for 

 obtaining the required lengths. 



The rod, the coefficient of expansion of which 

 is to be determined, is supported horizontally, and 

 is enclosed, with a thermometer tied to it, in a 

 glass jacket through which steam can be passed 

 from a boiler. One end of the rod presses against 

 the surface of a small fixed glass disc embedded 

 in a rigid vertical support. The other end of the 

 rod is free, and the expansion of the rod for 

 different ranges of temperature is measured, at 

 this free end, by means of a properly graduated 

 right-angled wedge faced with glass, and which 

 slides up and down against a firm vertical support 



Fig. i. — Co-epficiext oe Expansion Apparatus. 



The Simultaneous Rotation of a Current- 

 carrying Wire hound a Magnetic Pole, 

 and of the Magnetic Pole round the 

 Current-carrying Wire. 



In the narrow end of an inverted glass funnel 

 (fig. •">; is fixed an india-rubber plug carrying up- 

 wards a small hook to which is attached a short bit 



attached to the base. The initial length and tem- 

 perature of the rod are first ascertained. The 

 temperature is then gradually raised, and at several 

 intermediate steps the lengths are observed corre- 

 sponding to definite temperatures. The measure- 

 ments with this apparatus are most simple, only 

 two readings being simultaneously required, viz. 

 the thermometer and the wedge readings. 



