352 Prof. Ayrton and Mr. Mather on Galvanometers. 



Columns 15 and 16 give the deflexion per microampere and the swing per 

 microcoulomb, for the same controlling moment, 

 the resistance of each instrument being one ohm. 



)f 17 v 18 give the deflexion per microampere and the swing 

 per microcoulomb per cubic centimetre of coil, 

 when the period is 10 seconds and the resistance 

 of each galvanometer one ohm. 



„ 19 „ 20 give the deflexion per microampere and the swing 

 per microcoulomb per cubic centimetre of coil, 

 for the same controlling moment, and the re- 

 sistance of each instrument equal to one ohm. 

 Periodic time of any instrument is 10 vl seconds. 



Recent Improvements. 



Since the above-mentioned paper was published con- 

 siderable advance has been made in the construction of 

 sensitive galvanometers, more especially in the direction 

 of reducing the dimensions of the suspended parts. We 

 therefore thought it desirable to prepare a supplementary 

 list of records, giving the results obtained on more recent 

 instruments. In passing, we may remark that German and 

 American instruments of the suspended-magnet type show 

 the greatest progress in the reduction of dimensions and 

 consequent quickness of action, whilst the d'Arsonval, or 

 moving-coil type, seems to have received its greatest deve- 

 lopment in England. 



To make the present list more complete than the original 

 one, we have inserted three additional columns, giving 

 respectively the period of vibration of the suspended system 

 when tested, the logarithmic decrement, and the actual 

 sensitiveness of the instruments obtained in the test, reduced 

 to standard scale-distance and scale-divisions. From the 

 first two of these columns (Columns 1 and 2 of Table II., 

 pp. 356, 357) the relative suitabilities of the various instruments 

 for ballistic or deflexional observation can be seen ; whilst 

 the third column (Column 3, Table II.) shows the actual 

 " figures of merit " of the galvanometers (reduced to 

 standard scale-distance) in the condition under w r hich they 

 were tested. This latter column is of considerable importance 

 in the case of instruments whose controlling couple cannot be 

 readily altered— for example, in the case of ordinary moving- 

 coil instruments. 



In addition to the three columns above referred to, we have 

 also given in Table V. (p. 304) a list of records relating to 

 moving-coil galvanometers used as voltmeters. 



The values of D/r or D/r* in columns 9 and 10 of Tables I. 

 and II. express the relative sensibilities of the various instru- 



