COMPARISON OF PLATINUM AND CAS TIII.I;\K Ml. n i 51 



With the comparatively heavy magnet -system of this first galvanometer, we were 

 obliged, iii order to obtain the requisite sensibility, to work with a relatively long 

 time df vibration, which was not convenient for rapid work, and it was ultimately 

 t'< >ni ul necessary to so modify our galvanometer that the necessary sensitiveness should 

 IM- ohtained with a time of swing not exceeding six or seven seconds. We therefore 

 had recourse to the ingenious tyj>e of magnet-system devised by M. BROCA, of the 

 Ecole de MeVlecine, Paris, and descriliecl by him in the ' Journal de Pfaynqne,' 

 Keliruary, 1897. In its latest form this consists of two vertical hollow magnets, 

 having at their middle points a north and south pole respectively. When the two 

 similar but opjMwitely magnetised needles are fixed strictly parallel to one another, 

 the system thus formed is perfectly astatic in a uniform field, since the strength of 

 the p. ilc a i the centre of each magnet is equal to the sum of the poles at its ends, and 

 further the astaticism is not affected by even complete loss of magnetism in one of the 

 magnets. M. BROCA was kind enough to lend us a galvanometer of the type descrilx-d 

 in his paper, and a magnet-system made by himself. This instrument is the one 

 shown in the sketches of the galvanometer and suspension in figs. 5 and G. We had 

 not the means of accurately measuring the sensitiveness of this instrument by one of 

 the ordinary processes, but ascertained that, with a time of swing of five seconds, the 

 scale deflection, for a want of balance of the bridge corresponding to '001, was about 

 0'5 millim. This was with a system carrying a mirror large enough to give a bright 

 linage in the telescope, readable without difficulty in broad daylight. 



XIV. THERMOMETER LEADS. 



The thermometer leads were of stranded copper equal to No. 17 S.W.G., and about 

 seven metres long. The resistance of the four separate wires was carefully equalised 

 before fixing on the copper end-pieces used to make the contacts, each loop, P|P.., 

 (',( ' . ha\ ing a resistance of '15 ohm at 16. After nearly two years continual use the 

 two loops differed in resistance by '0003 ohm, a change of only about one-fifth 

 per cent, of the whole. 



The connections l>etween the resistance-box leads and thermometer were made by 

 means of the special alloy employed by Mr. GRIFFITHS, and from our own experience 

 we can strongly recommend these joints as very trustworthy and easy to make. 



XV. THE PLATINUM THERMOMETERS. 



The resistance of all except one of the platinum thermometers l>elonging to the 

 original Kew installation was such that their change of resistance between 0and 100' 

 \\as almost exactly one ohm. Though convenient for high range work, this type of 

 thermometer is hardly suitable for measurements of the highest accuracy at lower 

 temperatures, in consequence of the relatively considerable effect on the results of 



H 2 



