SCIENCE-GOSSIP. 



Wlmmmmmm 



CONDI CTED 11V JAMES QUICK. 



University of Birmingham.— The principal- 

 ship of the recently constituted University of 

 Birmingham Ikis been given to Professor Oliver J. 

 Lodge, D.Sc, F.R.S., Professor of Experimental 

 Physics, University College, Liverpool. Professor 



.1 II. Poynting, D.Sc., F.K.S., has been ainated 



Dean, in Dr. Lodge, Liverpool loses a man whose 

 remarkable energy and deep knowledge of physics 

 have more than preserved the required status of 

 the college. Professor Poynting's sphere of work 

 will not change, as he has hitherto occupied the 

 Chair of Physics at the Mason College, Birming- 

 ham. 



Electrical Discharges upon Photographic 

 Plates.— -Images maybe produced upon a sensi- 

 tive plate by means of an induction coil, using two 

 needles as poles. The positive needle should touch 

 the plate, the negative needle should be fixed about 

 half a millimetre away. A sheet of metal below 

 the plate will facilitate the action. Tne image 

 produced consists of numerous small black lines, 

 resembling arrows directed from the positive to 

 the negative pole. It is formed without develop- 

 ment, and is probably due to the fusion of the 

 silver bromide and of the medium, and after pro- 

 longed action to the reduction of the bromide. If 

 iodide of gold is substituted for bromide of silver, 

 the image consists of continuous brown lines con- 

 nected by many branches. 



Wave-Motion Model. — A striking method of 

 illustrating wave-motion is the following : — An 

 ordinary mercury tray about 70 > 50 x 10 centi- 

 metres has its smooth bottom painted white, and is 

 then tilled to a depth of about half a centimetre with 

 water slightly darkened with ink. The discoloration 

 of the water must be such that the bottom of the tray 

 is clearly visible through the layer of liquid when at 

 rest. If now a round vessel of about 8 to 10 centi- 

 metres diameter is placed in the traj r a circular 

 wave is set up spreading outwards. The motion of 

 this wave can be (dearly followed, as the thickness 

 of the liquid at the crest of the wave being so 

 much greater than in the trough, the appearance is 

 that <>f a dark hand travelling over a lighter back- 

 ii< i. Removing the disturbing cause give- a 

 •nd wave: and if the vessel is kept moving with 

 a period of one or two seconds, a train of waves i I 

 the same period is produced. .Many other interest- 

 ing effects can be shown, such as interference, 

 ction, and refraction. Plane waves can also be 

 produced by substituting a rectangular block for 

 the round vessel. 



Expansion of Air. a simple apparatus foi 

 the determination of the co-efficient of expansion 



of air can be arranged by taking a glass U-tube 

 •". or 1 millimetres bore, the limb containing the 

 air being aiiont 30 centimetres long and closed 

 at the end. The second Limb is about 50 centi- 

 metres long and open at the end. The bend of the 



tube contains strong sulphuric acid to a depth 

 hi L0 centimel res. Anotl >s-tube is slid 



into the longer limb of the U-tube and dips 

 into the acid. This tube serves as a plunger, and 

 is used to adjust the level of the acid to the same 

 heighl ii each Limb. The i im temperature 

 during experiments should not exceed 50 C, as 

 above thai value the vapour-pressure of the acid 

 would have to in- taken into account. The ■■■■ 

 however, has an advantage over mercury, as it 

 keeps dry the air under examination. The U-tube 

 is placed successively in water baths al 

 temperal ures, I he plunger being adjust* 

 time, and the measurement of volume made on 

 withdrawal from the bath. The results obtained 

 with the above arrangement are in close agree- 

 ment, even if made In elementary student.-. 



Wehnelt Interrupter. -Much work has hern 

 done in connection with this recently Lntrod 

 and valuable interrupter for induction-coil dis- 

 charges. Investigations have shown that the 

 interruptions are irregular, amounting to 17 per 

 cent, in the case of the original pattern Weh- 

 nelt instrument, and 24 per cent, with Sin 

 pattern. These irregularities have been discovered 

 in an interesting manner by taking a number of 

 mutoscopic impressions of the spark in the liquid 

 when the break was in action. It has thus been 

 shown that if a regular interruption is needed for 

 any particular class of experiments, the electrolytic 

 form must be discarded for a mercury -jet break or 

 a turbine arrangement. It has not been possible 

 until recently to use the Wehnelt break with a low 

 voltage, and various theories have been put forward 

 to account for this. Assuming that it is the oxygen 

 accumulating at the anode which prevents the 

 interrupter being thus worked, any process that 

 counteracts that accumulation must lower the 

 minimum E.M.F. required. This assumption has 

 been put to the test by making a jet of dilute 

 acid impinge against the electrode. With such 

 an arrangement 21 volts suffice- to obtain an in- 

 terrupted current of great steadiness. When the 

 pressure from the jet becomes too high, the current 

 becomes continuous. The impinging jet affords 

 the break another advantage, in that it keep- the 

 whole instrument cool 



Wavy ^park-Discharge. — Those who have 

 not seen the spark-discharge produced upon an 

 induction coil when worked in conjunction with a 

 Wehnelt break have an astonishing sight to 

 witness. The torrent of sparks produced and the 

 accompanying roar are radically different from tie- 

 discharge obtained with any form of spring or 

 mercury break. When the distance between the 

 points is reduced, the torrent of distinct sparks 

 changes to a solid flame, curving from one 

 terminal to the other. The frequency of the break 

 can be so increased that the note emitted by thi> 

 flame-discharge is quite shrill. Intermediate 

 between the above two effects, another peculiar 

 form of .-park i- obtained when tin- dischargers are 

 a point and a disc. It consists of a large number 

 of thin spark-lines branching out from the point 

 and extending toward- the disc like a brush. 

 Every one of the lines has a sinusoidal wave-shape, 

 due in reality to a spiral form of the spark-path. 

 When the sparks are successively photographed by 

 amutograpli.it is seen that t hey are very similar 

 itline, but that they are successively displaced 

 with respect to each other in the direction of the 

 disc. 



