Faraday to Jacob Herbert   19 March 1847

Royal Institution | 19 Mar 1847

My dear Sir

I now reply to your letter of the 4th instant¹ concerning the action of a central air reflector associated with the Rape oil lamp of Mr. George Herbert; the same lamp which without the deflector I reported to you in my letter of the 9th instant².

The flame of this lamp with the deflector is not so high as that of the Trinity Argand Sperm oil lamp which was used as a standard of comparison but it is wider and also wider than the flame of the same rape oil lamp without the deflector - a circumstance which is advantageous as increasing the horizontal divergence of the beam produced by placing the lamp in the focus of a parabolic reflector. The flame is also whiter and more intense than that of the Sperm oil lamp[.]

The flame can not be brought to its best condition at once but requires to be retained rather low at the first & then raised. In fact the cotton, from the greater heat upon it, contracts more when lighted than the cotton of the Sperm oil lamp and becomes deformed & hard if too much is raised at first. Gradually, the coal on the cotton thickens as a part of the oil is charred, & then the flame improves & if the wick be proportionately raised acquires its best condition. The sperm oil cotton may be raised to its best condition at once[.]

Much care is required in trimming the wick or else the flame soon forks and on the whole the lamp wants more attention than the sperm oil lamp - not because the wick requires trimming sooner but for the adjustment to the best effect & prevention of forking. This is very natural when one considers that, the glass being higher, the draught is sharper than in the Sperm oil lamp & the air is sent more directly towards the wick & the flame by the deflector[.]

I think the middle of the brightest part of the Rape oil flame (and also of the Sperm oil flame) is about 6/8 of an inch above the top of the burner (which is different in form in the two lamps) - and, that in that respect, therefore 7/8 is too much for the distance between the level of the reflector focus & the top of the burner.

In these experiments which were continued for several days I burned the lamps with equal light & in this case as before I found that more Rape oil was consumed than Sperm oil in the proportion of 1.18 to 1.00. In the former case i.e. without the deflector the proportion was 1.09 to 1.00. I believe it is in the shortness of the flame of the deflector Rape oil lamp that the cause of this loss of height in proportion to the fuel burnt is to be found[.]

Still the height was greater in this series of experiments than in the former & of course the consumption of oil greater. The consumption of oil for 24 hours was as follows

I ought to state that I estimated the lights of the lamps in a horizontal plane & in that position the deflector, as an opaque body in the middle of the flame, interferes least with it. When the eye is lowered, then the deflector interupts part of the light from the bright upper part of the further side of the flame and there is no doubt that when the lamp is in the focus of a reflector a considerable portion of light is thus cut off from the lower half of the reflector.

I have made these experiments very carefully and think both the Rape oil lamps good in their effects[.] Still I must observe that it can only be by an average of verifying practical trials at the lighthouses in the hands of different parties & for a protracted time that a precise estimate of their utility can be formed³.

I am My dear Sir | Your Very faithful Servant | M. Faraday

Jacob Herbert Esq | &c &c &c | Trinity House