426 PROFESSOR A. SCHUSTER AND MR. W. GANNON ON A 
were quite satisfied with the permanence of the covering obtained. The insulation 
was tested from time to time and the coil re-covered when necessary. When the coil - 
immersed in water was connected to one terminal of the battery, the second terminal 
being a bare wire, the resistance was generally found to be over 40,000 ohms, the 
electromotive force being 30 volts. That an insulation resistance equal to that 
amount is amply sufficient will be seen if it is remembered that leakage can only 
cause error in so far as the water is decomposed. If the whole of the current were to: 
pass through the water instead of through the coil, about 1 volt would be lost, so that 
an error of 3 per cent. only would result. Considering the small diameter of the wire 
and the high resistance of distilled water, it will appear that, even if our wire had been 
completely uncovered, the error introduced would not have been great, and even an 
insulation resistance of 1000 ohms would have made it inappreciable. 
The Stirring. 
The stirrer is shown in figs. 4 and 5. It consisted of a piece of brass foil cut to a 
half-moon shape. The upper surface was soldered to fine brass tubing, 17 centims. 
long. Two pieces of glass tubing fixed to the frame of the coil passed through 
circular holes cut into the stirrer and served as guides. The stirrer was moved up 
and down by a wheel and crank W (fig. 5) driven by an electromotor. A piece of 
brass tubing, of the same kind as that attached to the stirrer, was fixed to the 
crank, and the motion was transmitted by a flexible joint made of several layers of 
thin unvulcanised india-rubber, firmly tied to each of the two parts of the brass 
tubing. This joint, devised by Mr. Hapiey, worked very satisfactorily, and allowed 
sufficient lateral play to facilitate the adjustment of the calorimeter and coil below 
the driving wheel of the stirring apparatus. 
Some Questions concerning Thermometry. 
The method of using mercury thermometers in accurate work has undergone 
considerable change in the last twenty years. In measuring a temperature, the 
interval between 0° and 100° used to be determined by plunging the thermometer 
first into a mixture of ice and water, and then into boiling water. If the two readings 
are Ty, Tyo), the temperature ¢ corresponding. to an intermediate reading, T; would be 
t= 100 (I. — T/L ee 
Owing to certain properties of the glass envelope, analogous in their effects to 
elastic fatigue, a thermometer when exposed to a constant temperature will not, 
however, take up its final indication for a time which may be considerable, and this 
set of the glass affects especially the zero point, the apparent zero as at, first observed 
J. Sa ale be 
