XVI 



INTRODUCTION. 



the tin-foil project. No special expertness is then required on 

 the part of the experimenter to introduce these into the arc of 

 the current. The bulb can also be so placed in front that its 

 readings can be readily taken. The shorter end of the strip of 

 tin-foil, with the window, I place as is shown in fig. vi. In my 

 arrangement, the temperature of the strip of tin-foil rises in 

 almost arithmetical proportion to the number of elements 

 used,* when these are so arranged that each zinc is connected 

 with a copper pole. With one element, and the arrangement 

 just described, I obtain an elevation of temperature amounting 

 to about 5 C (9 Fahr.), and with six elements rather more 

 than 30 C (54 Q Fahr.). If great accuracy is required, the regu- 

 lation of the temperature must be accomplished by means of a 

 rheostat. 



In order to exercise a direct control over the temperature of 

 the glass cover, I attach a thermometer to the slide itself. In 



Fig. vi. 



All. 



A I. 



Gas chamber, with thermometer, capable of being heated by means 

 of a constant current. 



fig. vi., a represents the flattened bulb of the thermometer, whilst 

 the dotted line b indicates the direction of the tube. Both the 



* It must be expressly understood that the ratio here given corresponds 

 only to a certain definite arrangement. It follows from Ohm's law that 

 the resistance of the introduced strip governs this ratio. The strength of 

 the battery required must be ascertained by experiment. 



