﻿New Method for Mapping the Spectra of M etuis. 381 



rotates or vibrates, thus preventing welding and destroying 

 the coating of oxide which in some cases interrupts the 

 current between ordinary metallic poles. 



h Fig.l. 



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To accomplish this, a brass disk is fitted, by means of a 



collar and set-screw, to the shaft (or counter-shaft) of a small 



high-speed electric motor. Parallel to this brass disk, and 



upon it as a base, is screwed a similar disk. These disks are 



used as jaws in which to clamp small pieces of metal to be 



vaporized. One pole of the electric circuit which includes 



the arc is connected, by brushes, to the counter-shaft, shown 



in section at A (fig. 1). The other pole of the arc circuit is 



connected to another clamp F, which, by means of the screw 



E, can be made to approach or recede from the rotating disk. 



The clamp F is also fitted with parallel jaws, to receive a 



small piece of the metal (B) to be vaporized. This metal B 



is moved always parallel to itself, and the arc between B and 



C is maintained always at the same point. Both the rotating 



and the sliding jaws are mounted on the same base with the 



motor ; and the whole is so light as to be easily carried about 



in one hand. 



The disk is set in rapid rotation and the metal at B is slowly 

 fed in, by the screw E, until the arc strikes. The incan- 

 descent vapour is then carried out by the disk into the form 

 of an open fan, and is projected upon the slit of the spectro- 

 scope by the " image " lens. 



In the case of those elements which are easily obtainable 

 in the form of a regulus, an entire disk may be made of the 

 metal. With the rarer elements one needs to use only a 

 small piece in the clamp, but the time of exposure is cor- 

 respondingly lengthened. The disk once started, no attention 

 is required except the feeding-in of the metal, B. Nearly all 

 the wear is on this piece and very little on the disk, so that 

 the latter will last for a comparatively long time, while the 

 former has to be renewed with a frequency depending upon 

 the amount of current employed. We have generally used 



