PEIRCE. — CORRECTION FOR COUNTER ELECTROMOTIVE FORCE. 167 



The coil hangs from a single jewel, and the galvanometer is much more 

 sensitive than the " millivoltmeters " usually found in American labo- 

 ratories. The calibration curve for ballistic purposes is straight and 

 the scale practically one of equal parts; one of these instruments 

 which I have used has a resistance of about 60 ohms, and one micro- 

 coulomb sent impulsively through the coil causes a throw of 2 scale 

 divisions. This galvanometer was connected with a rheostat and the 

 low resistance secondary of a small induction coil, the primary of 

 which could be opened and shut by means of a commutator; a 

 rheostat in this primary circuit made it possible to change at pleasure 

 the amount of electricity set in motion in the secondary circuit. The 

 real resistance of the galvanometer circuit was about 74.9, and the 

 apparent resistance for all discharges up to 20 microcoulombs seemed 

 to be somewhere about 147. I took seventeen sets of observations with 

 different quantities, and got some results as low as 143.5 and others as 

 high as 151. The results all lay on a curious wavy line which seemed 

 to mark some idiosyncracy of this particular instrument. 



(I) The galvanometers used in (A) to (G) were all of the compact 

 coil form without the inside soft iron core of the original d'Arsonval 

 instrument. Galvanometers of this latter type, however, have their 

 own advantages, as is well known, and are of course in very common 

 use in commercial amperemeters and voltmeters and in laboratory 

 mirror instruments of moderate sensitiveness for use in bridge work 

 and in measuring ballistically condenser charges. As an extremely 

 satisfactory representative of this class I chose one of the Leeds and 

 Northrup Company's " Type H Galvanometers," of which there are 

 several in the Jefferson Laboratory. The calibration curve of this 

 instrument for throws due to condenser discharges is nearly straight, 

 for quantities up to about 4 microcoulombs ; the throw due to 1 micro- 

 coulomb being about 3.2 cm., whatever the resistance, up to 240,000 

 ohms, in the discharge circuit. A battery the electromotive force of 

 which was about 8 volts was made to charge — through a resistance 

 large enough (2000 ohms) to prevent possible polarization — a set of 

 standard condensers, which were afterwards discharged through the 

 galvanometer, and the throws corresponding to capacities of 0.1, 0.2, 

 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.00 microfarads, were about 2.75, 5.49, 

 8.21, 10.99, 13.82, 16.71, 19.70, 22.80, 26.02, 29.37. The "magneto- 

 inductor " and rheostat were attached to this galvanometer, and, with 

 the stops on the inductor rod in certain fixed positions, the throws of 

 the moving coil as measured on the scale were 15.81, 13.33, 9.76, 8.19, 

 or 7.05, according as the rheostat resistance was 0, 300, 1000, 1500, or 

 2000. The first of these numbers in connection with each of the 



