170 



and occasionally lead to errors of no small magnitude. The 

 errors of construction were described by Mr. Donovan, and 

 shown to be of such kind, that, in the attempt to preserve 

 sensibilitj'^, we sacrifice it by introducing a new source of de- 

 ception. A method of remedying these errors without such 

 a sacrifice was pointed out. Exceptions were taken against 

 the method at present in use for placing the galvanometer in 

 the magnetic meridian ; it was stated to be altogether inade- 

 quate ; and remarks were made on the influence of this error 

 on the indications of the instrument. An improvement in the 

 galvanometer in this respect was described. 



Galvanometers, as at present constructed, possess either 

 too much or too little directive power: in the former case they 

 are insensible to weak forces; in the latter they fail to indicate 

 strong ones, because the maximum effect is produced by very 

 feeble sources of power. The new galvanometer contains 

 different provisions against these imperfections. 



Several curious and important properties of the needle com- 

 monly called astatic were noticed ; and some consequences 

 highly detrimental to its proper action, and conducive to 

 greater error in its indications, were fully described. A new 

 instrument, called a volta-wagnetometer, was exhibited, which 

 detects and remedies these errors. A new method of mag- 

 netizing needles, and a new construction of needles, along with 

 several observations on the defects of those at present in use, 

 were introduced to the notice of the Academy. 



Instruments exhibiting the defects of the present con- 

 struction of galvanometers, as well as others in which these 

 defects have been remedied, lay on the table during the read- 

 ing of this paper. 



The conclusion of the paper was as follows : " The im- 

 provements in the construction of galvanometers here sug- 

 gested may be summed up as follows : 1 . the addition of means, 

 independent of the astatic needle, for setting the instrument 

 in the magnetic meridian ; 2. the close approximation of the 



