Chap. XL] POLARIS A TION OF ELECTRODES. 1 1 1 



muscles, nerves, or other structures are brought 

 into the circuit of the galvanometer, might produce 

 feeble electrical currents, which would cause deflec- 

 tions of the needle and would be erroneously attri- 

 buted to the tissue being examined. Such a source of 

 error is found in what is termed polarisation of the 

 electrodes. 



Polarisation of the electrodes. If two 

 platinum electrodes have been immersed in acidulated 

 water, and have been conveying a current for decom- 

 position, the positive pole will, after some time, be 

 found covered with bubbles of oxygen, while hydrogen 

 will be collected at the negative pole. If, now, these 

 electrodes be suddenly disconnected with the battery, 

 and connected with a galvanometer, the needle will 

 be deflected in such a way as to show a current in an 

 opposite direction to the original battery current. 

 This is due to the fact that the negative pole coated 

 with hydrogen becomes positive to the positive pole 

 coated with oxygen, This current naturally will 

 weaken the original current. This occurrence is called 

 POLARISATION OF THE ELECTRODES. Similarly, if a 

 nerve be laid across two copper wires, and a current 

 passed to the nerve, the electrodes will speedily be- 

 come polarised, and so sources of error will be in- 

 troduced into an experiment. In much the same way, 

 if a fresh muscle were to be connected to a galvano- 

 meter by means of copper electrodes, a movement of 

 the needle would be apparent at once ; but this might 

 be due simply to changes in the condition of the 

 electrodes produced at the two points of contact, and 

 not to any current obtained from the muscle. Even 

 very clean platinum electrodes would, after the lapse 

 of a little time, cease to be in precisely the same 

 electrical condition, and would thus give rise to electro- 

 motive force. To meet such objections, Du Bois- 

 Reymond constructed what are called NON-POLARISABLE 



