358 Prof. Ayrton and Mr. Mather on Galvanometers. 



than No. 24, but that No. 43 ivould be more than three times 

 as sensitive as No. 24 if the period of No. 24 was reduced to, 

 say, j 1 ^ of a second by increasing its controlling field, and the 

 period of No. 43 increased to -fa of a second. 



Comparison as Ammeters. 



On comparing Table II. of the present paper with the one 

 published in 1890 (shown in Table I., pp. 354, 355), it will be 

 observed that in columns 12 and 13 (which express the 

 factors of merit of the various instruments when time is 

 of importance) the values are on the average greatly in- 

 creased. For example, the highest values in the 1890 table 

 were given by one of Messrs. Elliott Bros.' Thomson instru- 

 ments of special form (see line 4, Table I.), for which D/r* 

 was 206*5 and D/r« 493, and these numbers were far above 

 any others in the list made out at that time. These figures 

 are now surpassed by no less than eleven instruments given in 

 the present list (omitting the oscillograph records in lines 4, 

 25 ; 46, and 47); and three of the eleven instruments are of 

 the moving-coil type. 



The highest values in the present table (omitting the 

 oscillographs previously mentioned) are those given by one 

 of Prof. Paschen's instruments, line 21, Table II., and are 

 5800 and 8750 for D/r 1 and D/r% respectively. We therefore 

 see that Prof. Paschen has produced an instrument which, at 

 a given periodic time and of a given resistance, is about 

 twenty times as sensitive as the best given in our previous 

 paper. Other instruments of the moving-magnet type giving 

 very high factors of merit are : — 



(a) Two Paschen instruments. (Lines 20 and 23 of table.) 



(6) Prof. Nichols' galvanometer*. (Line 11 of table.) 



(c) Smithsonian Institution instrument, designed by Wads- 

 worth t and constructed by Messrs. Elliott Bros. 

 (Line 15 of table.) 



(cl) Prof. W. B. Snow's bolometer galvanometer J. (Line 

 10 of table.) 



(e) Two galvanometers by Weiss §. (Lines 18 and 19 of 

 table.) 



To show at a glance the character of the improvement, the 

 numbers for these instruments are abstracted from the main 

 table and put in order of magnitude below. 



* « The Galvanometer,' by Prof. E. L. Nichols, p. 80. 



t Phil. Mag, vol. xxxviii. p. 553. 



X Physical Review, vol. i. p. 37. 



§ C.R. vol. cxx. p. 728, and Journ. Phys. vol. iv. p. 212. 



