Prof. Ayrton and Mr. Mather on Galvanometers. 

 Explanation of Table V. 



365 



Column 1 (or T) gives the periodic time of vibration of coil when 



tested (in seconds). 

 „ 2 (or X) „ logarithmic decrement of motion when 



tested. 

 „ 3 (or F') „ deflexion in divisions per microvolt when 



scale-distance = 1000 scale-divisions. 

 ,, 4 (or ;•) „ resistance of coil of instrument in ohms at 



about 15° C. 

 „ o (or ;•') „ resistance of instrument between terminals 



ar about 15° C. 



showed to the authors one of their high-grade instruments, 

 having a total resistance of 143 ohms, which gave 1*83 

 divisions per microvolt, its period being 12*1 seconds (see 

 line numbered 33, Table V.). Up to the end of 1896 the 

 ( Vompton instrument was the most sensitive d'Arsonval volt- 

 meter we had any record of, but its period was, for some 

 purposes, inconveniently long. 



Table VI. is an abstract of Table Y. arranged chrono- 

 logically, showing improvements since 1888. 



Table VI. 



Description. 



Date. 



Period Divisions 

 in per 



seconds. microvolt. 



Resistance 



of coil 

 in ohms. 



Total 

 resistance 

 in ohms. 



1. Invariable Sensibility 

 Type (Lame) 



1888 



1892 



1893 

 1896 



1897 



1897 



2-6 



2-2 



Aperiodic. 

 121 



5-84 



7'6 



0105 



1-31 



1-13 

 1-83 



(V55 



17 7 



21 

 132 



103-3 



22-2 



1-9 



57-5 



24-8 



178 

 143-3 



35-1 



5 75 



2. Ayrton-Mather Narrow 

 coil , 



3. Queen and Co., Phila- 

 delphia 



4. Crompton High Grade . 



5. Ayrton-Mather Narrow 



coil 



6. Ayrton-Mather Narrow 

 coil 









During 1897 several fairly low-resistance d'Arsonval in- 

 struments of the narrow-coil type were made at the Central 

 Technical College for use with the Lorenz apparatus tested by 

 Prof. J. Viriamu Jones and one of the authors for the McGill 

 University, Montreal. Records of two of them are given in 

 lines numbered 44 and 45 of Table V. and lines 5 and 6 of 

 Table VI. From these it will be seen that one gave 6*55 

 divisions per microvolt, and the other 17*7 divisions per 

 microvolt, the latter number being nearly ten times as great 

 as the previous best. Further particulars of these two very 

 sensitive instruments when used as ammeters are given in lines 

 44 and 45 of Table II. 



Phil. Mag. S. 5. Vol. 46. No. 281. Oct. 1898. 2 D 



