76 



Messrs. Ayrton, Mather, and Sumpner 

 Improved d'Arsonval Galvanometer. 



Deflexion from one 



Eelative Strength 



Error. 



Maximum Per- 



end of Scale. 



of Current. 



centage Error. 

















60 



60 









120 



119-9 



-o-i 





180 



179-8 



-0-2 





240 



239-9 



-o-i 





300 



299-8 



-0-2 





360 



360-2 



+0-2 





420 



420-5 



+0-5 





480 



480-7 



+0-7 



0-15 



540 



540-5 



4-0-5 





600 



600 









To improve the magnetic circuit in d'Arsonval galvanome- 

 ters some English makers replace the original tubular core 

 by a solid iron core ; and Messrs. Jolin of Bristol form the 

 core of a series of short horizontal magnets laid vertically one 

 above the other. A comparison that we have made of the 

 sensibilities of the instruments constructed in these different 

 ways has not shown that the English instruments are superior in 

 this particular respect to the French. This may be, however, 

 due to the fact that the horseshoe permanent magnets employed 

 by the English manufacturers are inferior to those used by 

 Messrs. Carpentier. 



The ordinary methods adopted for attaching the suspension 

 wires of the d'Arsonval coil to the supports seem to have been 

 designed with total disregard of the fact that these wires con- 

 vey the current into and out of the coil. This is particularly 

 serious when a d'Arsonval galvanometer is used as a volt- 

 meter, especially when it is employed to measure a small 

 fraction of a volt, and when therefore the total resistance in 

 the circuit must be small. 



In fact we have found that before any reliance can be 

 placed on the indications of a d'Arsonval galvanometer em- 

 ployed in this way, it is necessary to solder all the joints. 

 And instead of trusting to contact through the supports to 

 which the torsional suspension-wires are attached, we have 

 found it desirable to solder wires coming from the terminals 

 of the instrument directly to the hooks to which the suspen- 

 sion-wires are attached. Perhaps the best plan is to solder 

 wires at one of their ends to the terminals of the instru- 

 ment, and at their other ends to an extension of the torsional 

 suspension-wires. 



In devising very low-resistance d'Arsonval galvanometers 



