162 Dr. J. Hopkinson on High Electrical Resistances. 



A fine point of light was obtained from a fragment of a 

 silvered bead attached to the string with wax, and illuminated 

 by a suitably placed candle, and was observed in the direction 

 of the length of the string through an extemporized telescope. 

 In this way there could be no mistake as to the actual plane 

 of vibration, or uncertainty as to the direction of the wind 

 over the string. The path of the point of light was seen to 

 be nearly rectilinear and vertical, showing that the vibration 

 is across the wind. Sometimes the path was sensibly elliptic 

 with the major axis vertical. 



When a string is stretched across the slit at the bottom of a 

 slightly open window, there is usually some difficulty in de- 

 termining the actual direction of the wind where it plays upon 

 the string. On a still night, and with a regular fire, the 

 sound is sometimes steady for a long time, but it is wonder- 

 fully sensitive to the slightest changes in the draught. On 

 one occasion it was found impossible to open a distant door so 

 slightly as not to stop the sound, which would revive in a few 

 seconds after the door was closed again. A piece of paper 

 no larger than the hand thrown upon the fire (which was 

 burning without flame) altered the draught sufficiently to stop 

 the sound until the heated air due to its combustion had 

 passed up the chimney. It is the irregularity, and not, as has 

 been asserted, the insufficient intensity, of the wind which pre- 

 vents the satisfactory performance of the harp in the open air. 



Terling Place, Witham, 

 Feb. 8, 1879. 



XXVIII. On High Electrical Resistances. 

 By J. Hopkinson, F.R.S., JD.Sc* 



IN the Philosophical Magazine of July 1870 Mr. Phillips 

 describes a method of readily constructing very high 

 electrical resistances. A pencil-line is ruled on glass; the 

 ends of the line are provided with the means of making elec- 

 trical connexion ; and the whole is varnished : by this means a 

 resistance of two million ohms was obtained; and it was found 

 to be constant under varying potential. This method of con- 

 structing resistances is alluded to in Maxwell's ' Electricity ' 

 (p. 392) ; but I do not know that it has received the exami- 

 nation it deserves, or that it has come into general use. Having 

 need of resistances of over 100 million ohms, I have made a 

 few on Mr. Phillips's plan, ranging from 26,000 ohms to 

 96,000,000 ohms (which are fairly satisfactory), and one or two 



* Communicated by the Author. 



