460 Mr. R. L. J. Ellery on a Pendulum-Electrograph. [June 18, 



ments of a ring of brass of 4f inside and about 4 J outside radius, and f of 

 an inch wide ; they are firmly screwed to the ebonite so as to form together 

 a true arch, but with the end at the vertex separated by about -^^ of an inch, 

 care being taken that they are perfectly insulated from one another and 

 from all other parts. Attached to each electrode is a stout brass wire (p p') 

 projecting upwards and ending in a ring*. 



4. Charge-measurer. — On the aluminium stem of the pendulum, and 

 about two- thirds of the distance between the centre of the mirror and the 

 curved piece of aluminium from the former, is a disk of gilded mica {q), J an 

 inch in diameter with its plane parallel to that of the mirror ; fixed to the 

 outer metal case (to be described) is a sliding-tube and screw (r), through 

 which passes a steel wire («) terminating inside the case with a disk of sheet 

 brass 1 inch diameter. This is so arranged that the wire disk can be 

 pushed free through the sliding-tube and fixed with the disk at any defi- 

 nite distance from the mica disk ; while a further and gradual approach is 

 made with the sliding-tube and screw, both of which are divided ; the wire is 

 graduated to inches, the tube and screw together measuring to of an inch. 

 The large disk, when withdrawn, rests against the side of the covering case 

 6 inches from the needle. The screw is a four-threaded one with six turns 

 to the inch, and is sufficiently long to allow of 2 inches' motion of the 

 sliding-tube. 



5. Lifting -Cradle. — This consists of a cradle analogous to the lifting 

 apparatus of a good chemical balance. It lifts the pendulum at three 

 points ; one is a Y, a prolongation of the knife-edge, the other two are 

 formed by the arms of the balance-wire ; the cradle (M) is fixed to a piece 

 of ebonite (^) projecting from a tube {u) which slides on one of the pillars, 

 which can be moved up and down to the required extent by a screw (y v) 

 passing upwards through the bridge. This screw is worked by a key (w) 

 through the top of the outer case. The cradle is set upon the ebonite with 

 adjusting screws, so that it can be made to lift the pendulum quite symme- 

 trically, and lower it without setting it vibrating. 



6. The outer metallic cover. — x a? a? is a cylinder of copper 22 inches 

 high and 12 inches diameter, covered at the top. It has a flange (N) of 

 brass at the bottom which is ground flat and fits almost air-tight on the 

 slate slab ; three studs find their places in three holes in the slab and 

 keep it in position. In front of the mirror is the window (W) of the 

 cylinder, which is closed by a piece of parallel plate-glass, and covered out- 

 side by a metal plate having two tubular openings, to which are fitted the 

 telescope and dark channel for leading the slit of light to the mirror and 

 thence to the cylinder, as in the case of the Kew magnetographs. At the 

 back and opposite the window is the charging-hole (y) ; this is an opening 

 lined with ebonite and having a cap to screw on outside. Above the win- 

 dow, the sliding-tube and screw of the charge-measurer {r s) is fixed. On 



* Adopting SirW. Thomson's plan, I have since covered all the ebonite or vulcanite 

 parts with paraffin with great improvement in insulation. 



