Trof. Ayrton and Mr. Mather on Galvanometers. 363 



On comparing Mr. Duddell's 1897 instrument with those 

 of Messrs. Hotchkiss and Millis and Mr. McKittriek, we find 

 its factor of merit, as indicated by d/ri (the expression most 

 suited to instruments having comparatively low resistance), is 

 decidedly greater than either of the other two, in spite of the 

 fact of its wires being of phosphor-bronze instead of copper. 

 It may also be pointed out that the moment of inertia of the 

 moving parts of Mr. Duddell's 1897 oscillograph is very consi- 

 derably greater than the inertias of the two first-mentioned 

 instruments, being in the approximate ratios of 30 : 2*2 : 1, 

 thus showing the great superiority of the moving-coil type 

 for work of this nature, where short periodic time (or high 

 frequency) and good sensitiveness are necessary. In his 

 latest oscillograph Mr. Duddell has carried the reduction of 

 dimensions much further than has previously been attempted, 

 and has produced an instrument whose periodic time is 

 less than a ten-thousandth of a second (frequency 10,680). 

 Although its resistance is little more than 2 ohms, it actually 

 gives 420 divisions per ampere at standard scale-distance, and 

 its factors of merit, as represented by d/r% and d/vs, are 

 more than 30 times those of any instrument made previously. 



d'Arsonval Galvanometers as Voltmeters. 



In Table V. the actual sensibilities of various d'Arsonval 

 galvanometers when used as voltmeters are given, reductions 

 having been made to bring them to the same scale-distance 

 and scale-divisions only. 



As will be seen from the table, considerable advance has 

 been made in recent years. Of the d'Arsonval galvanometers 

 mentioned in our 1890 paper, the most sensitive instrument as 

 a voltmeter we had then seen was one of the " invariable sensi- 

 bility *" type, having a very large magnet. Its record is given 

 in line 4 of the d'Arsonval section of that table (see line 34. 

 Table I.). The instrument had a period of 2*6 seconds, coil- 

 resistance of 21 ohms, and total resistance of 57*5 ohms, and 

 gave a deflexion of 0*105 division per microvolt when the 

 scale-distance was equal to 1000 scale-divisions. Referring 

 to Table V. of the present paper, we notice that in 1892 a 

 narrow-coil galvanometer was constructed, having a period 

 of 2 - 2 seconds, coil-resistance 13' 2 ohms, and total resistance 

 24*8 ohms, which gave l'ol divisions per microvolt, an in- 

 crease of over 12 times in sensitiveness, with a shorter 

 period. 



In 1893 Messrs. Queen and Co., of Philadelphia, produced 

 a galvanometer of 178 ohms resistance, which gave 1*13 

 divisions per microvolt, whilst in 1896 Messrs. Crompton 



