115 



Some measurements on hysteresis and the effect of iron in tlie mag- 

 netic circuit have been undertaken. It would, however, be too premature 

 to take up their description at this time. 



The instrument which has been described was built, largely, by Mr. 

 Edwin Place, formerlj- connected \\ith the Institute. He made many 

 observations similar to those above recorded. 



I should like to take this opportunity to thank Dr. Gray for his many 

 suggestions and for the removal of a number of stumbling blocks. 



A Demonstration Apparatus. 

 By p. N. Evans. 



The apparatus is a simple modification of that commonly used to com- 

 pare, by diffusion, the density of another gas with air. It consists of a 

 porous battery-cell placed horizontally and fitted by a stopper to a glass 

 tube bent downwards at right angles a few inches from the stopper, 

 and then upwards again to its original height. This U-shaped manometer 

 is about two feet long and half filled with a dark-colored liquid; the limbs 

 are close enougli together to make a slight difference of level easily seen, 

 against a white background fastened to the tube. To further increase the 

 sensitiveness of the instrument a perforated glass plate or heavy card is 

 secured between two corks on the horizontal part of the tube close to the 

 cell, so that the cylinder or beaker of gas to be examined may be pressed 

 lightly against this, and thus largely prevent loss of the gas before suffi- 

 cient time has elapsed to show the maximum deviation in the manometer. 



While the ordinary apparatus is recommended for demonstration only 

 with gases differing considerably from air in density, this modification 

 has given very satisfactory results with hydrogen sulphide, and even 

 oxygen, with densities of 1.18 and 1.11 respectively, a difference in level 

 of at least an inch being observed in the latter case. A slight effect, 

 clearly visible to the manipulator, though not satisfactory for demon- 

 stration purposes, was obtained with nitrogen— density 0.972. 



