VII. RECORDING INSTRUMENTS. 587 



G. Cover for insolator. K. Battery. 



H. Clock. L. Punch. 



M. Reading-off apparatus : 



a. Drum. f.f. Stands for platinum wires. 



b. Graduated strip. g. Bunsen burner. 



c. Stand. k. Lens. 



d. Sodium carbonate. k. Spirit lamp. 



e. e. Platinum wires. 



Professor H. E. Roscoe, F.R.S. 



This method depends on the fact that the depth of colour produced on a 

 properly prepared chloride of silver paper is directly proportional to the 

 intensity of the light multiplied by the time of exposure. 



The apparatus consists therefore essentially of two parts, one in which the 

 prepared paper can be exposed for definite periods of time to the action of 

 the light, and a second part in which the intensity of the tint obtained can 

 be determined. 



The paper (A), previously salted by immersion for five minutes in a 3 per 

 cent, solution of sodium chloride, and cut into strips, is silvered by floating 

 for two minutes in a 12 per cent, solution of silver nitrate (B), contained in 

 the long trough (C), and afterwards dried on the reel (D). 



The prepared paper may be preserved either before or after exposure in 

 the dark box (E). It is next wound on to the bobbin of the insulator (F), 

 which is placed in electric communication with the clock (H) by means of 

 the battery (K) ; the free end of the paper, passing over the large wheel, 

 being held in position by means of a small pin on the inside circumference of 

 the wheel. 



When a current of electricity passes, the magnet attracts the armature, and 

 the wheel moves through a small space ; the circuit is immediately broken 

 and the armature released, and this slight movement of the wheel is repeated 

 every time a current of electricity passes through the apparatus. The insula- 

 tor is provided with a cover (G), in the top of which is a small circular hole, 

 against which the prepared paper is pressed by means of a spring, and as the 

 wheel revolves, fresh portions of the paper are successively brought under 

 this hole, and thus exposed to the action of the light. The mechanism of the 

 clock is so arranged that discs of prepared paper shall be exposed to the 

 action of the light each hour for 10 different periods of time, which have 

 been exactly determined, varying from 2 to 30 seconds, the object of this 

 being to obtain, either with the feeble light of the morning, or the strong 

 light of mid-day, a tint neither too light nor too dark to be read off. This is 

 accomplished by means of a large metal disc in the clock, which revolves 

 once in two minutes, and is in metallic connexion with one pole of the 

 battery. On the face of the disc are placed 1 1 platinum pegs, arranged at 

 equal distances from the centre of the disc, but at such different distances 

 from one another that the first 10 intervals correspond as closely as possible 

 to 2, 3, 4, 5, 7, 10, 12, 17, 20, 30 seconds respectively (their values being 

 afterwards experimentally determined to one-fifth of a second), whilst the 

 value of the last interval is of no importance. The other pole of the battery 

 is connected with a metallic lever tipped with platinum, the insulator forming 

 a part of the circuit. 



Each hour this lever is lowered mechanically so that it comes in contact 

 with the first platinum peg ; the circuit is completed, the magnet in the insu- 

 lator attracts the armature, causing the wheel to make a small fraction of a 

 revolution, and a fresh surface of paper is exposed to the light. Contact is 

 immediately broken, the peg passing away from under the lever, to be again 



