CREW AND BAKER. SPARK SPECTRUM OF CARBON. 



399 



4. After a variable (but definite and measurable) interval of time, 

 opeus a shutter in front of the slit of the spectroscope and exposes the 

 plate during any desired length of time, generally between \ second and 

 1 second. 



5. Interrupts the spark circuit. 



1. Again closes the arc circuit ; and so on, as before. 



The arc was operated with 15 amperes showing 40 volts between the 

 electrodes ; while the spark was produced by a large induction coil of 

 the type devised by Rowland in 1887 and described in Kayser's Hand- 

 buch der Spectroscopic, p. 183. This induction coil, or step-up trans- 

 former, was operated on a 104-volt alternating circuit, of frequency 120, 

 with a primary current of 20 amperes. In parallel with the spark gap 

 was placed a capacity of ^ microfarad. The arrangement of the circuit 

 is shown in Figure 2 where s x and s 2 are each double pole mercury 

 switches so fixed that one can be closed only after the other is opened. s x is 

 kept closed by a spring 



Carbon Electrodes. 



\ 



o 



until an electromagnet 

 begins to close s 2 by \_ 



3<^ 



< 



-o 

 s, 



-o o- 



Arc 

 Circuit. 



WMMMWV 

 MA/WLq 



-o o 



Spark 

 Circuit. 



FlGUKE 2. 



rocking a light beam of 

 which its armature is a 

 part. The question of 

 changing from arc to spark circuit is 

 then merely a question of closing the 

 battery circuit which actuates this elec- 

 tromagnet. This battery circuit is closed 

 and opened by a continuously rotating 

 switch (shown at the left in Figure 3) 



which is driven at the uniform rate of 10 R. P. M. by a small electric 

 motor. This rate of rotation is maintained constant by means of a pair 

 of coue pulleys and a heavy flywheel. 



This same rotating switch, or commutator, by means of the sliding 

 contact marked " 2 " in Figures 3 and 5, opens the shutter in front of the 

 slit of the spectroscope at any phase of the spark desired and holds the 

 shutter open for a small but definite period of time varying usually from 

 \ second to 1 second. On this same rotating commutator shaft is a stud 

 (D, Figure 3) which, immediately after the arc circuit is closed, pushes 

 a carbon rod into the arc gap for an instant and thus " lights " the 

 arc. 



By clamping the sector P (Figure 5) to the rotating commutator in 

 successive angular positions about its axis, one is enabled to open the slit 



