To James David Forbes   19th April 1852

Queenwood College n^r Stockbridge | 19^th April 1852.

Dear Sir,

I beg to return you my best thanks for the intelligence with which you have so kindly favoured me.¹

I send you a specimen of the wire of the rheostat which I use; its length is just sufficient to go once round the cylinder of the instrument – the latter is of serpentine stone and is a little over 4½ inches in length – a plate of brass $\frac{4}{10}$ of an inch in thickness is screwed on to each end of the cylinder and from the centers of these plates gudgeons protrude which fit into a pair of brass sockets and permit of the rotation of the cylinder. The wire coiled round the latter like a screw can be traversed from end to end by a small brass wheel with grooved rim, the point of contact of wheel and wire being the place where the current enters or emerges. The wheel moves along a brass rod and is pressed gently against the wire by means of springs to which the rod is attached; the rod is graduated to shew the number of revolutions and one of the brass plates at the end is graduated at its rim to shew the $\frac{1}{400}$ part of a revolution. It is a simplification of Jacobi’s instrument, and owes, if I mistake not, its origin to Poggendorff² – I purchased mine in Berlin the price was somewhat under three pounds – I hardly think you will get any thing of the kind at present in England.

The following rough sketch may throw a little more light on the matter: –

abcd the wooden base of the instrument

ef the brass rod.

g the brass wheel

hi,kl brass plates at ends – the latter graduated

m current enters – passes through gudgeon to the wire, and issues at g, thence to

n and forward through the other wire.

o the handle, by turning which g moves and any required length of wire is introduced.

But a sight of the instrument would be far more satisfactory than any description I could give – you will not I am sure deem me a forward fellow if I propose to you to take in Queenwood on your return to Edinburgh – There is 2 hours rail from Bristol³ to Warminster – between 2 and 3 hours coach from Warminster to Salisbury and half an hours rail from Salisbury to Dunbridge Station. This establishment is 4 miles from Dunbridge and is reached by a fly.⁴ London is three hours distant. If you could reconcile a visit to us⁵ with your arrangements I can truly say that everybody here would consider it an honour and I for my part would rejoice to shew you whatever little instruments I possess. I know that in writing this I take a certain liberty but I absolve myself from any approach to impertinence nor do I think that you will charge me with it.

I remain Dear Sir | your obliged & faithful Servant | John Tyndall

Prof. Forbes | &c. &c. &c.

StA JDF Incoming letters 1852, no. 42