Faraday to Jacob Herbert   26 January 1844

Royal Institution | 26th January 1844.

My dear Sir,

I found no sensible difference in the amount of oil consumed with the different cottons¹. The reason I believe to be as follows: the amount of oil which can be burnt with a clear bright smokeless flame depends upon the amount of air which has access to the flame. Now the burners and the lamp glasses were in all cases alike though the cottons were different, and when the lamps were lighted, the cottons were adjusted in height by the character of the flame, that being in all cases kept the same, the lamp glass was, by regulating the air, the regulator also of the oil to be burnt; and the regulation of that oil though supplied by wicks of different thickness &c is easily effected by the screw or rack. From the results it appears that any one of the cottons is able to supply oil enough for the kind of lamp glass used.

I have little doubt that if the lamp glasses were made larger and larger and the quantity of air passing through were continually increasing that a time would come when the thin cotton would fail to supply oil enough though the thick cotton would still do its duty.

The lamp glasses Mr. Wilkins supplied me with had a shoulder, and were much taller than those used in the lighthouses I have seen but he tells me that are such as are used in the floating lights. As there is more strength of draught through them than through the shorter cylindrical glasses, I have no doubt that the results obtained with them would hold good with the latter.

I am, My dear Sir | Very truly yours | (signed) M. Faraday

Jacob Herbert Esq | &c &c &c