380 Rev. F. J. Smith on a New Form 



the instrument consists of a heavy iron casting, C C (fig. 2) 

 fitted with a slide, B, the slide can be raised or lowered by 

 means of a screw with a milled head, the styli are attached 

 to the slide B. By raising the slide after each experiment a 

 large number of traces can be made side by side, and by the 

 addition of an extra carrier as many as 10 styli can be carried. 

 The pillar can be rotated on a triangular base, which is sup- 

 ported upon three levelling-screws. The screws rest after the 

 method of Sir W. Thomson, viz. in a hole, a groove, and on a 

 plane, on the slab which supports the chronograph. The 

 points of the styli are adjusted to the face of the glass by a 

 screw S. A simple pillar, A', can also be used when only one 

 set of traces is required. 



The tuning-fork. — A tuning-fork T is carried upon a pillar, 

 which slides upon a block in which a V-groove is cut ; a screw- 

 stop limits its position against the glass. The fork may be 

 driven electrically, or it may be excited otherwise. In the 

 experiments on explosions the fork was started by means of a 

 wedge, which kept the prongs apart, being suddenly with- 

 drawn by means of an electromagnet. 



The latency of electromagnetic styli. — While going through 

 a large number of papers upon investigations in which electro- 

 magnets were used for measuring time, it was found that in 

 nearly all cases it had been assumed that the armature of an 

 electromagnet was detached at the instant of the circuit being 

 broken ; also, that in two instruments of similar construction 

 the armature was released at the same instant. Several electro- 

 magnets of chronographs were examined : in some cases, where 

 much iron was used in the cores, the time between breaking 

 and release of the armature was as much as 0*04 second ; also 

 two apparently similar shaped electromagnets with similar 

 windings, equal currents being used, differed in their action by 

 some hundredths of a second. The experiments showed in a 

 very definite manner that some electromagnets were not to be 

 depended upon for close and accurate work. 



In order to produce an electromagnetic stylus free from 

 these evils a large number of experiments were gone through 

 on the relative proportions of the cores and their yoke, also 

 upon the winding. It was found that the yoke should be made 

 large as compared with the cores ; the dimensions finally 

 adopted were :— 



Cores .... 2 millim. diam. ; 10 millim. long. 



Yoke .... 20 millim. x 5 millim. x 5 millim. 



The method of measuring the period of delay spoken of, 

 which may be called the latency of the stylus, is as follows, it 



