568 STATE BOARD OF AGRICULTURE. 



The galvanometer has at least four advantages over the telephone. 1. 

 The direction of the deflection indicates on which side the bridge is un- 

 balanced and the exact balance can be made easily and quickly. On the 

 other hand in tlie case of the teleplione tlie sound increases equally with- 

 out distinction for a departure from the setting of the resistance in either 

 direction from that which gives a true balance. 2. When the current is 

 put on, the direction of "creeping," if there is any, will indicate whether 

 there is polarization or lieating. Tliis will quickly show where and how 

 the changes in the sj^stem are to be made. 3. It is only necessary to close 

 the switch for less than a second at a time when finding a balance. Con- 

 sequently the possible error due to heating or polarization is negligible. 

 When using a telephone the current is on all the time until the balance is 

 found. 4. The eye is more accurate than the ear and even if the two 

 instruments had the same figure of merit the one that makes use of the 

 eye is preferable. The operator is not under a nervous strain even if he 

 desires to attain an accuracy of .001 of !%■. 



The galvanometer costs more than the telephone but it is a well known 

 fact that in general, increased sensibility in the indicating device leads 

 to a reduction in size and cost of all the rest of the equipment required. 



RESISTANCES. 



The ordinary resistances and even the best grades of coils put out 

 until recent years are not suitable when measuring the resistances of 

 dilute solutions of electrolytes providing accurate results are desired. 

 This is so because the coils are not free from capacity and inductance. 

 These are the two great sources of error to be guarded against when using 

 alternating currents. In a system like the Wheatstone Bridge every 

 conductor has a capacity especially when it is surrounded by other con- 

 ductors and this capacity can be defined as the ratio of the charge of 

 the conductor to the potential produced by the charge. Its value is de- 

 pendent on the form and dimension of the conductor and the nature of 

 the dielectric. The electrolytic cell and the resistances act as capacities 

 and vary with the current period. High resistance coils give the larger 

 error. With resistance coils of 20,000 ohms or more, the power factor is 

 large on account of the poor dielectric which the resistance contains. 

 Consequently the capacity is large. 



The general effect of capacity is to produce a "lead" in the phase and 

 whenever the phase difference is large the currents are apt to be in quad- 

 rature and in this case though a current passes through, no deflection of 

 the galvanometer scale is produced. Therefore correct resistance values 

 can only be obtained by meeting the condition that the four arms of the 

 bridge are without appreciable capacity and inductance. The new re- 

 sistance coils are now wound to avoid capacity and inductance. The 

 capacity of the electrolytic cells is eliminated by introducing a condenser 

 of proper value. Kohlrausch (14) drew attention to this matter some 

 time ago, but it is only in recent years that our knowledge of condensers 

 has sufficiently advanced to develop one that is efficient. The coils in the 

 bridge should also be wound to avoid capacity and inductance. 



The effect of unbalanced capacity is to make the "balance point" less 

 precise. The "balance point" is taken as that point where there is no 

 deflection. This means that the unknown solution in the cup just bal- 

 ances the known resistance. This definition further assumes that a move- 



