230 



SCIENCE-GOSSIP. 



SOME NEW PHYSICAL APPARATUS. 



By James Quick. 



(Continued from page 198.) 



A Gold-leaf Electroscope, with Special 

 Ebonite Insulation. 



A TUCH inconvenience and annoyance is very 

 frequently caused to a lecturer when, having 

 prepared an elaborate set of experiments on static 

 electricity, he finds that, due to dampness or other 

 causes, his gold-leaf electroscope at the last moment 

 becomes quite unreliable, and will not hold its 

 charge for more than about half-a-minute, with 

 the result that his lecture is a partial failure 

 from an experimental point of view. Electroscope 

 experiments illustrating Faraday's laws of equal 

 inductive charges demand that the electroscope 

 shall not lose any of its charge during the whole 

 time of the experiment, while it is found quite 

 impossible to make quantitative measurements 

 upon relative specific inductive capacities if one 

 has an untrustworthy instrument. The present 

 pattern, made to Professor Chattock's design, is 

 intended to overcome these troubles and also 

 to be a useful instrument for direct lantern 

 projection on the screen. Its efficiency depends 

 principally upon the insulation afforded by a 

 special ebonite arrangement for supporting the 

 gold leaves. 



The construction of the instrument is seen 

 from fig. 3, and fig. 4 shows a sectional view 



Fig. 3. — Gold-leaf Electroscope. 



of the ebonite insulating part to which I have just 

 referred. A rectangular arch of zinc is screwed to 

 a wooden base, and is provided with two parallel 

 glass sides held in position by small removable 



brass lugs. The electroscope is thus adapted for 

 projection, as it can be placed directly in front of 

 the condenser of a lantern, and the 

 image of the gold leaves focussed D 



upon the screen by means of an 

 ordinary objective. Down through 

 the centre of the top of the arch 

 slides an ebonite rod (E E) supporting 

 the gold leaves, etc. This ebonite 

 rod is drilled from both ends to about 

 half-an-inch at the centre (A B), and 

 a smaller hole (C) is drilled through 

 the centre-piece. This smaller hole 

 carries the brass rod (D) supporting 

 the leaves, which are thus only in 

 contact with half-an-inch of ebonite 

 on the inside of the rod. The only 

 source of leakage, therefore, is up 

 the brass rod to the centre of the 

 ebonite rod, and either through the 

 substance of this or along its inner 

 surface to the outside, and over this 

 to the metal arch supporting it. The 

 result is that the charge is very con- 

 stant upon the leaves, and can be Insulation. 

 retained there for days. The sensi- 

 tiveness is increased by making the gold leaves 

 thin and narrow, and by keeping the electrical 

 capacity of the instrument small. A binding-screw 

 is fixed to the supporting arch for properly earth- 

 ing it, and a small hole drilled in the top brass 

 plate for hitching on wires from the various instru- 

 ments under experiment. 



An Adjustable Cathode X-Rav Tube, 

 Magnetically Controlled. 



When working with the ordinary form of X-ray 

 tube, two difficulties present themselves. Firstly : 

 under different conditions of working and different 

 spark lengths, the one tube with its one degree of 

 exhaustion and one value of resistance cannot be 

 adapted. Secondly : upon continued working, it 

 is found that the exhaustion, and therefore the 

 penetrating value, of the tube increases, so that 

 finally, in spite of repeated heatings by a Bunsen 

 flame or other source, to increase the pressure 

 inside, the resistance of the tube becomes so high 

 that the electrical discharge will not take place 

 under the same conditions for which the tube was 

 originally selected. 



These difficulties have been overcome, chiefly 

 owing to the persistent patient work of Mr. A. A. C. 

 Swinton, whose results upon the modus operandi 

 in the interior of the tube, also upon the conditions 



