[sHAW-REiLLEY] EFFECTS OF AGEING 175 



ently few cells maintain a value of about 100 microvolts difference 

 from the mean. 



§4. Notes on the Repairing of Bad Cells 



The remarks recorded in this last section are based in some cases 

 upon the observation of only a few cells, and are mentioned therefore 

 with some reserve. As other bad cells were not available and as there 

 seemed to be no immediate probability of supplementing these obser- 

 vations, it was thought of sufficient interest to add these conclusions 

 in their present form. The following list enumerates what loosely 

 may be termed some of the main "diseases" of standard cells with 

 their "symptoms" and "treatments." 



1. High Internal Resistance (a) With Hard Packing. If the 

 internal resistance becomes much greater than 1000 ohms, it is found 

 that the sensitiveness of a potentiometer circuit in which the cell is 

 used, is obviously lowered. This trouble is due to the hard packing 

 of the crystals and paste during a period, or repeated periods, of 

 prolonged slow cooling {e.g., in a thermostat after use at 25°C. in a 

 cool room). A gentle application of heat at a temperature not higher 

 than 30°C., followed by rapid cooling while being slightly shaken, has 

 proved an effective treatment in several cases. The shaking must 

 not be violent enough to dislodge the paste, {b) With Gas Bubbles. 

 These are rare in the Weston, but very common in the old Clark. 

 The same treatment as in (a) was found effective in the only Weston 

 cell in which this defect occurred. 



2. Results of Short- Circuiting. The e.m.f. is lowered, but if it 

 has not been affected by more than a certain limiting amount which 

 appears to vary with different cells, a long period of charging with a 

 current of approximately the same magnitude as that obtained on the 

 occasion of the short circuit, will remove the trouble. 



3. Results of Exposure to Higher Temperatures. If the e.m.f. has 

 changed appreciably from this cause it does not seem possible to repair 

 the cell. Apparently a permanent inversion has taken place. 



4. Acidity. This is due to some contamination at the time of 

 construction, the effects of which often remain latent for months. 

 Possibly the acid is included in the crystals or localized in part of the 

 paste, and thus has at first no appreciable influence on the behaviour 

 of the cell. Green feathery streaks often appear in the paste and 

 occasionally in the packed part of the crystals. The cell should be 

 opened and made more basic. If the CdS04 is to blame, a cure may 

 be effected, but if it develops that the whole of the paste is affected, 

 the trouble cannot be remedied unless the cell is practically remade. 



