538 



Mr. P. E. Shaw on 



into 500 parts, the screw-pitch was \ mm.;, the graduations 

 were subsequently corrected against dead-hard pieces of steel 

 5, 10, 15, &c. mm. long, verified by the Standards Department 

 of the Board of Trade, the errors being very small. 



The three outer legs of the spherometer were removed and 

 the frame (AB) of the spherometer was fastened firmly to the 

 thick board (XX) (fig. 1), then the end of the screw (S) 



Fig-, l. 



abutted against the long arm (CL) of a lever which was held 

 against it by a spring, while the short arm (CM) carried a 

 small brass rod holding in its end a platinum wire beaded 

 at the end (a). 



The telephone-receiver (Rec) is firmly fastened in the 

 board (XX) so that the centre of the diaphragm, which 

 carries a small platinum plate, is opposite to the platinum 

 bead. Then the circuit, consisting of the cell (Z), the re- 

 sistances (R), the observing-telephone (T), the key (K), and 

 the contacts, is completed when (a) and (b) meet, which can 

 be brought about by a forward or backward movement of the 

 screw (S). 



As seen in the figure, the graduated disk (G) has a 

 pulley on the back of it ; a cord passes round this and a 

 series of other pulleys, so that the screw can be worked to 

 and fro by the observer without touching the screw directly 

 by hand. By the use of the pulley-string, vibrations are 

 not so readily transmitted from the hand, and, moreover, by 

 that means the observer can work from a convenient dis- 

 tance, reading the disk (G) by a telescope. 



The telephone (T) has long flexible wires, so that the 



