Dec. 14, 1888.] 



SCIENTIFIC NEWS. 



601 



SIMPLE EXPERIMENTS IN PHYSICS. 



AS a means of illustration, nothing can excel projection 

 by means of a good optical lantern. Not only can 

 pictures and diagrams be shown clearly to a large 

 assemblage, but apparatus of various kinds may be 

 projected on a mammoth scale ; many chemical actions 

 may be exhibited ; the phenomena of light, heat, elec- 

 tricity, and magnetism may be shown in various ways. 

 In fact, there is scarcely a branch of physics that may 

 not be illustrated in this way. The lantern is becoming 



Fig. i.— Arago's Experiment. 



deservedly popular in colleges and schools and for pri- 

 vate use. Besides being of great use for general instruc- 

 tion, it affords a means of rational amusement and 

 entertainment. A poor lantern, like any other inferior 

 piece of apparatus, is undesirable. An instrument for 

 scientific work should have a triple condenser, a recti- 

 linear objective, a swinging front for the vertical attach- 

 ment, a calcium or electric light, polariscopic and micro- 

 scopic attachments, an erecting prism, and an alum or 

 water tank. Such an instrument may now be pur- 



Fig. 2.— Magnetisation by Means of Spirals. 



chased tor a reasonable price, so that there is no 

 economy in making one's own instrument. It will, how- 

 ever, be found advantageous to make attachments. A 

 simple way of illustrating Arago's experiment, showing 

 the magnetising effect of an electric current on soft iron, 

 is represented in fig. 1. The lantern to which this 

 and other pieces of the apparatus are adapted is provided 

 with two rods, projecting from the front of the instru- 

 ment and connected with binding posts, which in turn 

 are connected with a battery of dynamo. The base of 

 this apparatus is furnished with spring clips for engag- 



ing the conducting-rods of the lantern. .To the upper 

 ends of two posts rising from the base are attached the 

 extremities of a copper wire, which is bent into spirals 

 at its fixed ends. The wire is bent twice at right 

 angles, and is curved downwardly between the arms 

 extending from the spirals. The ends of this wire are 

 connected with the clips. On the base below the curved 

 part of the wire is placed a box well filled with iron 

 filings. The box and wire are projected on the screen, an 

 erecting prism being used. The wire is pressed down- 



Fig. 4.— Incandescent Lamp 

 Projection. 



ARRANGED FOR 



ward into the filings, and withdrawn before the current 

 passes, to show that the wire, uninfluenced by the cur- 

 rent, is not able to lift the filings. The current is sent 

 through the wire when it is again dipped into the filings. 

 This time it will take up a quantity of the filings, as 

 shown in the engraving, each fragment of iron becoming 

 a magnet, which tends to place itself at right angles to 

 the current. When the current is interrupted the 

 filings fall. In fig. 2 is represented a device for show- 

 ing the magnetising effect of a helix, also the different 



Fig. 3.— Sturgeon's Magnet. 



results secured by helices wound in opposite directions. 

 The frame is provided with metal clips for attachment 

 to the rods of the lantern, and two helices which are 

 oppositely wound with respect to each other are 

 stretched across the frame. The ends of the helices are 

 connected with the clips, so that the current passes from 

 one clip through both helices, as indicated by dotted 

 lines, to the other clip. The helices are provided with 

 a coating of insulating varnish. A darning-needle is 

 placed in each helix, and when no current is passing a 

 magnetised cambric needle, suspended by a fine thread, 



