VIII. BIOELECTRIC MEASUREMENTS 



243 



The Calomel Electrode. This typo will maintain a constant 

 and reproducible potential if only reasonable precautions are taken 

 in preparing the electrode. For this reason, it has been extensively 

 used for potential measurements in biology. It will even maintain 

 a reasonably constant potential during current flow, which is a 

 very stringent requirement. 



There are a number of different satisfactory forms of the calomel 

 electrode, one of which is shoAvn in Figure 3. A platinum wire sealed 

 into tube A makes contact with the mercury at the bottom of the 

 tube. This is covered with a paste made by grinding mercurous 



Fig. 3. Calomel reference electrode ma)- 



chloride with mercury together with a small amount of molar potas- 

 sium chloride in a mortar. This is covered with a molar solution of 

 potassium chloride saturated by prolonged shaking with mercurous 

 chloride. Contact is made to the liquid under investigation by means 

 of tube C. The potential of this electrode is +0.282 v. at 25° re- 

 ferred to the standard hydrogen electrode as zero. The plus sign indi- 

 cates that the electrode is positive to the solution. 



If it is necessary to maintain a constant potential in the face of a 

 current flow, the success achieved will be roughly proportional to the 

 area of the mercury in contact with the solution. Electrodes as 

 large as 8 cm. in diameter have been used. 



