1883.] On the Electrical Resistance of Carbon Contacts. 13 



9 ohms ; but with 400 ohms inserted between the battery and bridge, 

 a pressure of *35 grm. gave a resistance which on different occasions 

 varied between 73 and 110 ohms. When once, however, the resist- 

 ance has been reduced by a strong current, the changes which occur 

 when feebler currents are used are comparatively small — unless, of 

 course, the points of contact are first separated. Of the nature of 

 such changes I shall say more presently. The following table gives a 

 few measurements, using one Leclanche cell : (1) with a resistance of 

 300 ohms between the cell and the bridge ; (2) without the 300 ohms. 

 It will be understood that the bismuth cylinders were separated from 

 each other every time the weights were changed. The effect in ques- 

 tion is most marked with small pressures ; with comparatively high 

 pressures it quite disappears. 



Table X. 



Resistance of bismuth contacts 



. A 



( \ 



Pressure. w ith 300 ohms when the 300 



between battery ohms is 



and bridge. removed. 



Grms. Ohms. Ohms. 



•5 60 1-9 



2 5 37-2 14 



5 5*4 17 



7 5 7 7 2 



10 -8 -8 



15 6 5 



25 14 -14 



12. Adhesion of Contacts due to Current. 



This phenomenon is accompanied by another. After the passage 

 of the current which has caused the resistance to be so remarkably 

 diminished, the bismuth cylinders are found to adhere to each other, a 

 quite appreciable force being necessary to separate them. This effect, 

 which is common to all metals, has been thoroughly investigated by 

 Mr. Stroh, a description of whose experiments is given in the "Journal 

 of the Society of Telegraph Engineers," vol. xix, page 182. I shall, 

 therefore, say nothing more about it, except that I have succeeded in 

 causing two pieces of very thin platinum wire to stick to each other 

 with a current of about a milliampere. Mr. Stroh attributes the 

 effect to fusion. 



13. Effect of Variation of Current after a strong Current has passed 

 through the Contact. 



A third apparent anomaly is the following : a small weight being in 



