Electrical Method for Measurements. By P. E. Shaw. 627 



As an instance of another class of measurement the following 

 is given. If it be desired to accurately measure -the number of 

 lines in a diffraction grating used to produce a normal spectrum, 

 place the grating on the stage and proceed as before. A table of 

 results is given. 



Every 50 Lines in a Diffraction Grating. 



llirrometer 

 Reading. 



Difference in 

 Term of M . 



225 



G79 



1134 



} 



7Q 1 



1593 

 2040 

 2498 

 2953 

 3410 



454 



455 

 459 

 453 

 458 

 455 

 457 



Mean 456 /j. for 50 lines. 



Fig. 127 shows the micrometer by itself. 



Fig. 128 shows the whole apparatus for Microscope measure- 

 ments. 



Details. — This method of measuring by electric contact has 

 been exhaustively tested for several years and is quite reliable. 

 Consistent results can be obtained if two conditions are observed: — 

 (1) Both the contact surfaces of the screw and the stage must be 

 metallically clean ; fine emery cloth should be used on them till 

 they are quite free from lacquer, oil, &c. (2) Vibrations should 

 be avoided. Mount the apparatus on a steady table not subject 

 to vibrations, and handle the screw delicately when taking a 

 measurement. Any cell will do for the circuit, and any telephone 

 receiver will act, though a " loud speaker " is most convenient. 

 For resistance I have used a few hundred ohms. The object in 

 having resistance is to keep the current small, but it is not an 

 essential. In completing the circuit through the Microscope, I 

 have used any spare screw on the Microscope and brought the 

 joining wire to it. If there is no screw convenient, the wire may 

 be simply lashed firmly on the instrument, but of course to an 

 unlacquered surface. It should be observed that the method 

 works equally well whether immersion is adopted or not. 



2 u 2 



