68 Mr. A. Stansfield on some Improvements in 



A galvanometer, designed to be as free as possible from this 

 defect, was made by Dr. Muirhead ; the coil was shuttle- 

 shaped, instead of being hollow as in the ordinary form, and 

 was suspended by a strip at the top ; the connexion at the 

 bottom was made by a fine spiral. This instrument returned 

 to its zero-reading fairly well after small deflexions, but for 

 large deflexions it was less reliable than the older form ; 

 it was, moreover, much more susceptible to vibrations, a fact 

 which greatly reduced its usefulness, as it was impossible to 

 obtain a reasonably smooth photographic record of its readings. 

 All the records shown were obtained by the open-coil galva- 

 nometer. 



The Recording Apparatus, shown at H in fig. 1, consists 

 of a photographic plate I, mounted on a float F, which slowly 

 rises as water is admitted into the cylinder H. Vertical 

 glass rods at the sides of the cylinder act as guides to prevent 

 rotation or lateral movement of the float. The whole is 

 covered by a hooi K, which can readily be raised by means 

 of the pulley and counterpoise. It is supported by a "hole, 

 slot, and plane,'"' on the fixed board R, with which it 

 makes a light-tight joint. A slit S, which forms part of the 

 hood_, permits only horizontal beams of light to reach the 

 plate ; consequently it is possible to have the room fully 

 illuminated without danger of fogging the plate. The source 

 of light L is an ordinary glow-lamp enclosed in a wooden box. 

 A brass tube, with a rectangular diaphragm at the end nearest 

 the lamp, cuts off all the light except that from a selected piece 

 of its vertical filament. Light from this filament is reflected 

 by means of a plane mirror on the galvanometer, and focussed 

 on the photographic plate by a lens in front of the galvano- 

 meter. 



This method of focussing "the light, which was suggested 

 by Prof. Boys, enabled a fine line of light to be obtained for 

 recording, notwithstanding the fact that in order to obtain 

 records on a sufficiently large scale the galvanometer was 

 placed 16 feet from the photographic plate. This line of 

 light passing through a horizontal slit immediately in front 

 of the plate produces a square spot on the plate. The glow- 

 lamp is very convenient for ordinar}^ work, but the light is 

 insufficient for taking rapid records, and it becomes necessary. 

 in taking such records, to employ a lime light. 



'The Thermo- Couple consists of two wires, one of platinum 

 and the other of platinum alloyed with 10 per cent, of rhodium 

 or iridium. The wires are fused together at the end and 

 inserted for protection into a thin fire-clay tube. Each wire 

 Wc's insulated from the other, except at the junction, by means 



