to the Construction of a Standard Galvanometer. 419 



As the galvanometer was very dead-beat no calibration- 

 experiment need take longer than 20 seconds, so that this is 

 without effect. 



In order to use the instrument the cell is coupled up in 

 series with a platinoid resistance of 1417 legal ohms; the 

 galvanometer-coil being itself 2*02 ohms, the cell about 5*5 

 ohms. Consequently the current used in testing is 



1-4262 nmnA7 



C= 1417 + 2-02 =QQ1QQ7ampere - 



This is quite near enough for our purpose. 



In order to set up the galvanometer once for all the follow- 

 ing dispositions were made. The galvanometer-coil was 

 pushed up towards the suspended magnet as far as it would 

 go ; this was known as position 0. The controlling magnets 

 were then raised so as to give a mean sensitiveness ; the 

 known current was put on and reversed and the double de- 

 flexion noted. This deflexion was indicated by a fiducial 

 mark on the scale. If at any time the sensitiveness of the 

 instrument changes, it is only necessary to bring the coil up 

 to its position and raise or lower the controlling magnets 

 by means of the adjusting arrangement till the deflexion 

 reaches the fiducial mark. 



When the coil is in the position A, '001 ampere corre- 

 sponds to 10 scale-divisions ; and at B, to 1 scale-division. 

 These points were found by using a very large storage cell, 

 whose E.M.F. was tested whilst the calibration was going on, 

 and employing suitable resistances from a thick wire box. 

 The whole arrangement was tested by this means from '001 

 to '4 ampere, and it was found that the results were wonder- 

 fully consistent : this was possibly in part due to the mirror 

 not being very good, and consequently the observation is so 

 far wanting in accuracy. With the rather bad fight-spot the 

 readings could not be taken nearer than to about 1 per cent, 

 at the end of the range, and 3 per cent, towards the centre. 

 Of course if currents of only three or four milli-amperes are 

 to be measured, these can be got with at least this accuracy 

 by using the coil at some convenient point near O, A and B 

 being rather far away. As an accuracy to about 5 per cent. 

 is all that is requisite in measuring currents for hospital work, 

 there can be no doubt that this instrument fulfils the purpose 

 for which it was made, having an accuracy in use of at least 

 five times this amount. I have to thank the assistants in my 

 laboratory for the excellency of their workmanship. 



