William Snow Harris to Faraday   22 December 1860

6 Windsor Villas | Plymouth 22 December 1860

My dear Faraday

Did it ever occur to you to consider, what difference there is, if any between the state of a coated Plate of Glass charged in the usual way directly from the Electrical Machine - and a Plate of Glass excited on one side with a coating on the other side only - and then a second Coating applied after the manner of the Electrophorus and an Experiment by Cigna¹.- To make myself very clear Let Fig 1 represent the First case .. Fig 2 the excitation case by Friction over an under coating B. Fig 3 the subsequent application of an upper Coating A - It appears to me that the latter case Fig 3 is really not the same thing as Fig 1² - or is it a charge at all of the system in the way of Fig 1 The small Spark which is perceptible on joining A and B suppose B insulated is so far as I see a mere affair of the two coatings - If I unite A and B Fig 1 by a bent metal Wire - I neutralize or discharge that system - and I can get nothing more out of the Cover A except so far as some small residuary charges may be obtained in the way you have described Expt. Researches 1246³ - But in the Case of Fig 3 when I unite the coatings AB by a metal wire I merely equalize the forces between the Coatings whatever may be the consequent change in the intervening Dielectric - I do not discharge the system as in Case Fig 1 - This is seen by the continued bearing off Electricity by Induction through the Instrumentality of the Cover[.]

In Fig 1 we charge & Polarize the system by Foreign aid as it were. In Fig 3 & 2 we merely develop the Electricity of the Glass itself - There is certainly some difference here which it might be well to elucidate[.] Now with respect to your Expt. sec 1246 I have no doubt of the accuracy of your observations but I think nevertheless there is more yet behind.- Beccaria⁴ says that in a compound Glass Plate of 2 laminae Fig 4 - coated as before; each ½ plate will be + on one Side and - on the other.

I have been lately studying with very much attention this subject. I have repeated all the Experiments of Cigna, Symmer⁵ & others - I have certainly found with you that most commonly on separating the Plates one Plate is + without any negative Induction - the other - without any positive Induction-

But all these sort of Experiments are very precarious + I think in situ one surface must be + & the other - because the two plates a & b Fig 4 cohere & that forcibly - now as opposite Electricitys attract it follows that the near surfaces must be in opposite Electrical states - It is the act of separation which disturbs the condition[.]

You can certainly have a glass Plate + on one side & - on the other - Now let me just remark, that it does not follow because we find on presenting each side of an Electrified Plate of Glass to an Electroscope - the Electroscope is similarly influenced by both sides, that therefore both sides of the Glass are Electrified - suppose I excite a disc of Glass on one surface only - that excitation may as easily influence an electroscope through the Glass on the one side as through the air on the other - (I do not consider here the question of Specific Inductive Capacity.) - nay more the side or surface opposite the excited surface may have a slight negative state but which you cannot detect because the positive Electricity of the opposite Surface (suppose the excitation +) prevails & conversely Well as I said - you can obtain both + & - coincident surfaces if you will balance or equalize the force Let HH - annexed Fig 5 be an insulated Glass disc about a foot in Diameter AB two moveable Coatings or covers about 7 Inches diameter set upon vertical Insulators & moveable Bases NN so that the Coatings may be withdrawn on each side from the Glass, let ab be two Electroscopes of divergence connected with the coatings AB. Charge either side A in the usual way, giving side B, a temporary communication with the Ground. Then Electroscope a on the charged side diverges - but Electroscope b exhibits no Induction - connection with Ground being withdrawn - still we can not doubt but that the whole Dielectric is Polarized - In fact if we withdraw the Coatings Electroscope b diverges with neg Electricity whilst a is Positive - But you get no negative Indication from the Glass - both sides appear to give positive Electricity - but that is because the Positive Electricity of the excited side A predominates.

Replace the Covers & repeat the charge - Electroscope a being divergent and b tranquil - remove the connection of b with the Ground - and now by an Insulated metal Ball G of an inch Diameter abstract Electricity from the predominant side A by degrees Electroscope b on the negative side begins to rise - go on untill the divergence of a & b is alike or nearly so again withdraw the Covers: one is + the other - of course - but now the glass surfaces exhibit the same opposite states one is + the other –[.] That is suppose you present one to Gold Leaves divergent with either Electy., (say Positive) the one side of the Glass increases the Divergence whilst the other causes the Leaves to close[.] But I must not tax your patience further – Avogadro⁶ has written most capitally on this subject - he draws a distinction between Electricity of the Coatings - Electricity of the Glass &c. &c Nicholsons⁷ Journal - Octavo - Vol 21 page 288⁸[.]

believe me to be | My dear Faraday | always yours most sincerely | W. Snow Harris

Dr. Faraday