SCIENCE 



[N. S. Vol. XXVII. No. 679 



a small sewing needle, and giving wonder- 

 ful sensibility. Thomson was the first to 

 insist on the advantage of small size in 

 magnetic and other measuring instruments, 

 and his galvanometer became the model of 

 all delicate galvanometers from that time 

 to this, each increase in lightness having 

 been attended with an increase of sensi- 

 tiveness. It is interesting to recall, in con- 

 nection with the first cable, which lived to 

 transmit only 732 messages, that it was 

 ruined by the practical, that is non-theo- 

 retical, electrician, Mr. Whitehouse; who 

 applied to it currents from huge induction 

 coils, probably giving potentials of two 

 thousand volts. By the advice of Thom- 

 son, thus dearly paid for, this was reduced, 

 on the 1865 cable, to a few volts, this being 

 amply sufficient to work his delicate in- 

 struments. The mirror galvanometer, to- 

 gether with the electrometers invented be- 

 fore the cable was talked of, were the fii'st 

 of Thomson's many electrical measuring 

 instruments, by which he will perhaps best 

 be remembered by practical people. Later 

 he invented the siphon recorder, still in use 

 for recording cable signals. Thus Thom- 

 son became the first, as he was the greatest, 

 of electrical engineers, telegraph engineers 

 then, but now embracing the many fields 

 of telegraph and telephone, wireless, and 

 transmission of power. In this connection 

 may be mentioned his connection with the 

 establishment of practical units for all 

 electrical measurements, first made impera- 

 tively necessary by the cable, and lying at 

 the basis not only of all exact measurement, 

 but of all practical engineering. A com- 

 mittee of the British Association for the 

 Advancement of Science was appointed, 

 ■with Thomson as chairman, to consider the 

 question of units in general, and in 1863 

 they made the determination of a practical 

 unit of resistance, now known as the ohm, 

 the method of experimentation being de- 

 vised by Thomson. Finally the efforts of 



this committee culminated in the proposi- 

 tion of what is known as the C.G.S. sys- 

 tem of absolute measurements for every 

 sort of physical quantity, this system being 

 now in use by every scientist and electrical 

 engineer in all parts of the globe. 



We may now say a few words of Thom- 

 son's instruments. Of the galvanometer 

 we have already spoken, this being the 

 most easily understood of his instruments, 

 and used for measuring current. For the 

 measurement of potential he devised two 

 electrometers, acting on the principle of 

 the attraction and repulsion of statically 

 electrified bodies. In one, the absolute 

 electrometer, a horizontal plate was hung 

 from the arm of a balance, which weighed 

 the attraction due to a parallel fixed plate. 

 This idea was again not invented by Thom- 

 son, but by Snow Harris, who, however, did 

 not understand how to get correct results 

 with it. The bright idea conceived by 

 Thomson was to surround the disk with a 

 so-called guard-ring, the idea being that 

 as the calculation supposed an infinite 

 plate, the suspended disk should be, as it 

 were, a sample of a larger plane surround- 

 ing it, the disk alone being movable. Thus 

 the demands of theory and practise were 

 both met at once, and exact calculation be- 

 came possible. In the quadrant electrom- 

 eter a quite difi'erent, but equally origi- 

 nal arrangement was adopted. The mov- 

 ing part was made in the shape of a hori- 

 zontal figure of eight, and turned about a 

 vertical axis, most delicately supported by 

 a silk fiber, and attracted and repelled 

 by a circular box enclosing it, cut 

 into quarters, which alternately attracted 

 and repelled, but so as to combine their 

 action, giving a most delicate instrument. 

 These instruments, like the galvanometer, 

 have become classical. By his journeys on 

 cable ships, and his practical experience as 

 a yachtsman, Thomson devoted much 

 thought to the needs of navigation, and 



