136 REPOET— 1893. 



Dr. von Helmholtz expressed his full concurrence in these decisions, 

 which are, as he informed the Committee, in accord with the recommenda- 

 tions which have already been laid by the Curatorium of the Reichs- 

 anstalt, as well as by himself before the German Government. 



APPENDIX II. 



Experiments on tie Effects of the Heating produced in the Coils hy the 

 Currents used in Testing. By R. T. Glazebeook. 



Various circumstances (notably the experiments of Mr. Griffiths ') 

 had made it appear probable that the heating effect in the coils produced 

 by the current used in making the resistance test might be sufficient to 

 affect the results of the tests. Some experiments were made to examine 

 the point directly. 



The resistance of a coil of 100 ohms (nominal value) was measured in 

 the usual way, i.e. by making a Wheatstone's bridge of four coils whose 

 nominal values were "l, 10, 10, and 100 ohms. If the coils had been 

 accurate there would have been a balance ; as it was, one of the 10-ohm 

 coils needed to be shunted, and the adjustment was made by determining 

 the value of the shunt when no current passed through the galvanometer. 



As the current in the battery circuit was increased by varying the 

 number of cells this shunt decreased in value, showing that the effect of 

 the heating was to produce an apparent diminution of the resistance of 

 the 1000-ohm coil. This, of course, is as would be anticipated ; for \S^ of 

 the current goes through the 1-ohm and one of the 10-ohm coils ; the re- 

 maining yV goes through the 10-ohm and the 100-ohm. The rise 

 of temperature will clearly be greatest in the first 10-ohm coil, and to 

 counterbalance the increase in resistance produced thereby it becomes 

 necessary to reduce the shunt. 



The following readings were obtained : — 



The true value of the 100-ohm is given by taking the product of the 

 values of the two 10-ohm coils at the temperature of the observations, 

 dividing by the value of the 1-ohm and multiplying by a factor repre- 

 senting the effect of the shunt. 



During the above observations the temperatures remained steady, 

 the factor changed from 1 — ^00028 to 1 — -00034. Thus the resistance of 

 the 100-ohm coil changed by -034 -•028, or -006 ohm. 



The ajiparatus was not sensitive with a smaller current ; the effect, 



' PMl. Trans., 1893. 



2 Only one observation at this current was made ; the others are the mean of 

 several. 



