484 Transactions. — Miscellaneous. 



penetrate, any depth, a cylinder of thin copper was placed 

 over the needle. The needle gave no appreciable deflection, 

 showing that the copper cylinder completely screened off any 

 effect on the iron. A thin external iron cylinder gave the 

 same effect. 



In order to determine with accuracy the state of a needle 

 which had been under the influence of discharge, recourse was 

 had to a method of solution of the iron. After several pre- 

 liminary experiments, dilute HN0 3 at a temperature of boiling 

 water was found to give the most reliable results. In order 

 to test the rate at which the iron was eaten away a piece of 

 pianoforte-wire 6-5cm. long, 0-032in. diameter, was taken and 

 placed inside a solenoid, and subjected to a steady field of 

 100 C.G.S. units. The needle was then assumed to be mag- 

 netized uniformly throughout its section. 



Plate XLVIIL, Fig. 2. 



E HFis a glass vessel, inside another glass vessel, 

 A B C D, which is supported on a tripod of copper. Water 

 is kept boiling in the outer vessel by a burner, K. Inside the 

 inner vessel, but not touching it, is the needle, firmly fixed by 

 the ends in a light frame. This frame is supported clear of 

 the vessels by the stand, S. 



The needle is fixed horizontally at a distance from the 

 magnetometer, E, to give a convenient deflection on the 

 scale. As the water is heated up to boiling-point the 

 deflection due to the needle decreases slightly, due to the 

 effect of temperature on the magnetic moment of the 

 needle. 



At a stand alongside, the dilute HN0 3 is kept in a beaker 

 of boiling water, and when all is ready the HN0 3 is quickly 

 transferred to the vessel EHF, taking care not to disturb 

 the needle. The moment the HN0 3 reaches the level of the 

 needle in the vessel the time is noted, for at that instant the 

 needle commences to dissolve. Sufficient HN0 3 is poured 

 in to cover the needle half an inch. 



As the needle is dissolved the deflection falls, and the 

 deflection at different intervals is carefully noted. 



This method of fixing the needle first and then pouring iu 

 the acid was unavoidable, as the maximum deflection due to 

 the needle could not otherwise be obtained. By keeping the 

 HNO, at 100° C. and rapidly transferring it to the vessel 

 (itself surrounded by boiling water) we insure that the needle 

 is covered by HN0 3 at the same temperature during the 

 whole time of solution. Since the amount of acid was large 

 compared with the size of the needle, the effect of solution of 

 the iron would not materially alter the rate at which the 

 needle was dissolved. 



