A YEAR OF THE X BAYS. 661 



vacuum. These are called adjustable vacuum tubes, and afford 

 a means of controlling the requisite sparking gap of the coil 

 within certain limits. Nos. (4) and (5) are now almost the only- 

 styles of tubes that meet with favor. 



Three types of apparatus have been employed in exciting the 

 X rays. All are necessarily such as are capable of producing a 

 high electric potential, and all were in use prior to Dr. Rontgen's 

 discovery. They are the Ruhmkorff induction coil, the plate in- 

 fluence machine (either the Wimshurst or the Topler-Holtz), 

 and the Tesla coil. The only development in these machines has 

 been in some instances the improvement of their quality and en- 

 largement of their capacity without, however, introducing any 

 novelty in the type of the apparatus, unless we except making 

 the condenser of the induction coil adjustable in capacity. The 

 most suitable rate of interruption of the primary current for each 

 coil and tube may best be found by trial. Where a continuous 

 current is supplied from a commercial circuit of a hundred and 

 ten volts or more a rotating segmental wheel as interrupter with 

 a rheostat in circuit is of advantage, but many experimenters get 

 as good results by using a storage battery of six to ten cells, with 

 an ordinary hammer break in the coil. The Ruhmkorff coil is 

 used to give a unidirectional discharge in the Crookes's tube. In- 

 fluence machines having several rotating plates act in the same 

 way, and with excellent effect. Tesla coils are employed to give 

 exceedingly rapid discharges to and fro in the tube, which re- 

 quire, therefore, two terminals that can both act as cathodes. It 

 can not be said that either of these three forms is per se the best. 

 With proper accessories one will give as good results as another, 

 but the ordinary induction coil with suitable single-focus tube is 

 the most generally practicable. 



Fig. 4 shows an outfit of apparatus for X-ray use. It consists, 

 in this instance, of a variable rheostat connected to one main of a 

 hundred-and-ten-volt continuous current ; in series with this is, 

 next, a rotary interrupter, which is also driven by a current from 

 the same main circuit ; then comes an ammeter, then a pole 

 changer or reverser, which connects back to the other main and 

 forward to the primary of the large Ruhmkorff coil. This coil 

 has in its base a condenser which is united with an additional 

 adjustable condenser. There are, further, a double- focus tube, 

 fluoroscope, and screen. 



The most obvious suggestion of usefulness for the new agent 

 was in surgery. It was so easy to discover any foreign substance 

 in portions of the body, or to perceive the nature of any bony 

 malformation, that it was hoped that surgery had received a valu- 

 able assistant in these rays. From time to time reports of suc- 

 cessful operations based upon such revelations have been made. 



