PEIRCE. — BALLISTIC GALVANOMETERS OF LONG PERIOD. 285 



from the throw due to the same amount of electricity sent impulsively 

 through the coil when at rest in its position of equilibrium. 



The galvanometer I sought did not need to be very sensitive, but it 

 must have one property which, according to my experience, is rare in 

 suspended coil instruments ; that is, there must not be the slightest 



Figure A. 



This electromagnet lias a laminated core made of sheet iron one third of a 

 millimeter thick and weighs about 300 kilograms. 



sensible shift of the zero point due to thermal currents or to chemical 

 action at the junctions when the galvanometer circuit should be closed 

 on itself. This condition forbade the leading of the current into the 

 galvanometer coil through the phosphor bronze or steel gimp by which 

 the coil was suspended, and required that the whole galvanometer cir- 

 cuit, even to the binding posts and connectors, should be of one metal, 

 copper. 



It is of course not desirable to make the period of a ballistic galva- 

 nometer long by making the righting moment due to the suspending 

 fibre small, for a weak fibre makes the zero point uncertain, and a large 

 throw on one side usually shifts the zero point slightly in that direction 

 unless the gimp is even stouter than that commonly used in sensitive 



