270 Mr Griffiths, The Calibration of a Bridge Wire. [Feb. 11, 



We may assume that under these conditions the rate of fall 

 in the E. M. F, of the two cells S^ and S^ will be the same. (In 

 practice I have found that a rough approximation in the total 

 resistance of the two circuits is sufficient to secure the necessary- 

 equality in the rate of fall.) 



Having decided as to the minimum lengths of AB which are 

 to be compared, place two contact makers on the bridge wire 

 (as at dj and Pj) separated by (approximately) this minimum 

 length. Connect one of the wires from the contact maker P^ with 

 a high resistance galvanometer (G) and then with the circuit G^, 

 to some portion of which it should be soldered (at P^). The wire 

 from d^ must pass directly to the second circuit and be shifted along 

 it until G shows no deflection ; it should then be soldered in place 

 in order to prevent any movement during the subsequent opera- 

 tions. 



The contacts c?, and P, are then moved along AB, their 

 distances being re-adjusted whenever G shows a deflection. 



If a reversing key is placed at K the final calculations are 

 simplified by proceeding as follows. Place c?, at A and adjust P^. 

 Next lift dj over P^ towards B, reverse K, and then adjust d^ 

 leaving Pj unaltered and so on. Thus lengths of equal resistance 

 can be determined without any overlapping of the parts thus 

 found, a matter of great practical convenience. 



The only contacts moved throughout the operations are those 

 at d^ and P^, and also those (if the reversing method has been 

 adopted) of the key at K. All these contacts are potential ones 

 and therefore any variation in them is of no consequence. It 

 is advisable to place the resistances Pj and R^ in the same tank 

 of water or paraffin, and thus the results will be practically 

 unaffected by changes in their temperature. 



The contact d^ should occasionally be placed in its original 

 position, and if the reading of P^ is then found to be unaltered we 

 have proof that the rate of fall in the E. M. F. of the two cells 

 is identical. If any alteration is shown a time chart can be con- 

 structed in the manner usually adopted when comparing cells by 

 the Poggendorf method, and any corrections introduced which 

 may be found necessary. 



The accuracy of the calibration thus performed depends on 



(i) The potential gradient down AB. 



(ii) The sensitiveness of the galvanometer. 



(iii) The accuracy with which it is possible to measure the 

 length c^jPj. 

 It is not advisable to make the potential gradient steeper 

 than is necessary, as the inequalities of the wire would be exag- 

 gerated by any excessive rise in temperature, that rise being 



