440 SCIENTIFIC EXPERIMENTAL APPAEATUS. 



12. EowlancVs absolute electrometer for potentials represented by sparks 

 of about 0.1 to 1 inch. Designed on Sir William Thomson's guard -ring 

 principle, and constnicted by Edelmann, of Munich. Guard-ring, 33.5 

 centimetres, and can be separated about 7 centimetres, the distance 

 being read by vernier to 0.01 centimetre. Movable disk, 10 centimetres 

 diameter, and firmly attached to arm of a T)alance sensitive to 1 mgr. 

 Balance moves only 0.01 centimetre, and means of two distances of the 

 disks are taken, the one to move it to upper and the other to lower stop. 

 Weights of from 1 to 5 grammes ordinarily used. Dislcs ground and 

 polished to mirror surface after niclcel-plating. 



13. Boutland''s electrostatic standard condenser. — Constructed by Grunow, 

 of N"ew York. One sphere within the other nicJcel-plated and ground to 

 mirror surface^ with extra ball for interior. Balls, 7 and 8 inclies diam- 

 eter. Hollow sphere, 10 inches diameter. Apparatus for centering. 

 Eadii determined by loss of weight in water. Can be charged and dis- 

 charged any number of times at rate of three per second, by means of 

 hue wires which pass in momentarily from outside, and so do not change 

 the capacity much. Any condenser can be comj^ared with it by means 

 of an electrometer. 



14. Thomson^ quadrant electrometer^ by White, of Glasgow, with 

 Thomson's key. 



15. Condenser., 1-3 microfarad, by Elliott. 



16. Commutators for high (say 1 inch si)ark) as well as low tension. 



17. Rowland's galvanometer for the absolute measurement of discharges 

 of high tension. Constructed by Eowland & Schneider. Coils wound 

 with paper between, and boiled in paraflQne in vacuo at 100° C, to be 

 thoroughly dry. Needle shielded from electrostatic action, and deflec- 

 tion read by mirror and scale. Number of coils about 11,000. Constant 

 on the cm. gr. second system 19091. =G of Maxwell, as determined by 

 comparison with galvanometer described in American Journal of Science, 

 Vol. XV, p. 334. See No. 27, below. 



18. Boivland's absolute galvanometer for the measm^e of quick, weak 

 currents. Constant, 1833.2. See American Journal of Science, Vol. XV, 

 p. 334. Constant very accurately known. 1,790 turns. Can be used as 

 sine galvanometer or with mirror and scale. Horizontal circle reads to 

 1', but is readily estimated to 30". Telescope and bar for determining 

 horizontal intensity in exact position of instrument. 



19. Eozcland's tangent galvanometer, brass circle, 50 centimetres diam- 

 eter. Circle graduated to 15' and 20 centimetres diameter. From 1 to 

 243 turns can be used, the constant of each set being known with great 

 accuracy. INIade by Meyerstein, but altered and wound and coils meas- 

 ured by Eowland. 



20. Two Thomson^s galvanometers of high and low resistance, the first 

 differential with coils around both needles and set of shunts. Made by 

 Elliott, of London. 



21. Tico Nobili astatic galvanometers, by Elliott, and one by Sallerou. 



