82 



KNOWLEDGE. 



March, 1911. 



blade or blades. It is immaterial whether the jet 

 revolves or whether the copper teeth revolve around 

 the jet, the r&sult is the same. The mechanism is 

 enclosed in a glass or iron vessel, which contains a 

 quantity of mercurv at the bottom, and then filled up 

 with spirit or paraffin ^il. The break thus taking 

 place under the surface of the fluid is all the more 

 sudden and complete, and this t\pe of niercurx 



break has been, and is. 



very popular. The great 

 disadvantage is the large 

 quantity of mercury re- 

 quired (twenty to thirty 

 pounds in some) and the 

 rapidity with which the 

 latter becomes emulsified 

 and useless for the time 

 being. Most of the mercurx" 

 can be recovered, but the 

 process is very messy. 

 (See Figure 3.) 



Owing to this difficult)' 

 of cleaning, efforts were 

 made to find some gaseous 

 medium to replace the 

 liquid in common use, and 

 it was found that ordinary 

 house gas was all that could 

 be desired : and it nia\- 

 be said that an\' niercurx 

 break designed for the use 

 of liquid will work equalh 

 well, or even better, if the 

 gas is used instead. The 

 case has, of course, tu he 

 made tightlv-fitting so as 

 not to allow free escape 

 of the gas. Used in this 

 way the mercury does not 

 become emulsified, only a 

 small quantity is required, and the small amount of 

 black mercury compound that gradualK' forms need 

 onl}' be removed at long intervals. 



The most recent development of the mercur\- 

 break is a more or less radical departure from the 

 jet type we have been considering, but the change 

 is a very important one, and the svstem upon which 

 they work is one that is likel\- to prewail. The prin- 

 ciple involved is an old one, though its ajiplication 

 to interrupters is quite new. It is well known that 

 if we fill a hollow sphere with liquids of different 

 densities, and then rotate the s()here rajiidly. the 

 various liquids will tend to arrange themseKes 

 around the equator of the sphere with the heLi\'iest 

 liquid against the wall and the lightest li(]uid 

 nearest the centre of rotation. Ajjiilying this 

 principle to an interrupter, the hollow sphere is 

 flattened at the poles and the equator is bulged out. 

 This form is found to give the best results, as might 

 be supposed. (See Figure 5.) Into this jar is placed 

 some mercury and paraffin oil, and the whole is 

 mounted upon the end of the shaft of an electric 



Gy tJte ctiiirttsy of 



Figure 2. 



motor, which is placed vertically. As the latter is 

 set in motion the jar turns with it. and the mercurw 

 by virtue of its greater weight, at once takes up its 

 position at_^the widest part. Its rate of rotation is 

 a little less than that of the jar. which is made of 

 cast iron, both for its strength and its resistance to 

 the action of mercur\-. There are several forms of 

 interrufiter working on this principle, but the above 



arrangement forms the 

 basis of them all. In one 

 of them, a fibre disc with a 

 metal segment, and about 

 the size of a five-shilling 

 piece, is mounted so that 

 its edge engages the whirl- 

 ing band of mercurv. This 

 causes the disc to rotate 

 rapidh. and as the metal 

 segment touches the 

 mercury the circuit is 

 completed, to be suddenly 

 and completely broken 

 w hen it leaves. 



The ■' make "' and 

 " break " in other modifi- 

 cations of this type need 

 not be described in detail ; 

 they are by far the most 

 efficient of the mercury 

 breaks, and the mercury is 

 not emulsified and used u() 

 anything like so rapidh". 

 Electrolytic breaks are not 

 so much used now as they 

 were some time ago. These 

 work on an entireK' different 

 principle and are the sim- 

 plest in construction of all 

 of them. They require a 

 great deal of current, and 

 the effect on the X-ray tubes is rather severe, but many 

 radiologists jirefer them to any other form. Their 

 action is fully explained in most te.xt books on elec- 

 tricitx-. but is too technical for an article of this kind. 

 When we consider the remarkable property of 

 the X-ra\s in reatlih' passing through substances that 

 are quite opaque to ordinary light it would seem that 

 such (lught to be of the greatest use under manv and 

 diverse conditions. In the early days of their 

 discovery manv extravagant predictions were made 

 as to their probable value and these were treated 

 more or less seriously. As a matter of fact, if we take 

 awa\- their application in medical and surgical work, 

 so little remains that the demand for the necessarv 

 apparatus would be so small as to be unworthy of the 

 serious attention of an\- manufacturer, e.xcept in the 

 fulfilment of a special order. 



It will be thus easih' understood how difficult it is, 

 when treating of the uses of the X-rays,' to a\i)id 

 reference to medical matters. Their limited use in 

 other directions is due to several causes, and the chief 

 one is that the X-ra\' image is a silhouette and not a 



Messrs. X,-.ot,m if Co. 



I'l.itinmii Break. 



