914 



Voltaic Electricity. Section 4: Ohm's, Kirchhofi's and Joule's Laws. 



No. 61873 



6 1 873 A. Fi-ont View. 1:8. 



61 873 B. Back View. 1 : 8. 



61,873. School Rheostat after Kolbe, Figs. A and B (Ztschr. f. d. phys. u. chem. U. 21, t <' 

 1908, p. 217), with supplementary rheostat and table of resistances on the back . 7. 0. 



Fig. A shows the front of the apparatus. Upon a white lacquered, vertical wood slab is arranged ; 

 first of all a single ohm of bare constantan wire, which can be regulated between and 1 ohm without 

 intermediate steps, by means of a reliable sliding contact. The resistance values corresponding can be 

 read off on an easily interchangeable cardboard scale. This cardboard scale is interchangeable in order 

 that it can be arranged properly when the constantan wire has undergone some wear. To the right of the 

 single ohm is a switch handle rheostat of constantan wire, consisting of 20 spirals of 0.5 ohm each, covered 

 green, and two spirals of red-covered manganin wire (visible on the back) of 5 ohms each. All spirals 

 can be got at easily by the employment of push terminals and can easily be re-calibrated should this 

 prove necessary after long use. All connections are laid open, being comprehensively arranged. The single 

 ohm and the switch handle rheostat are connected in series but can also be used independently. All values 

 between and 21 ohms can be got in fine stages by means of two handles. 



On the back of the rheostat (Fig. B) is fixed a table containing the resistances of the commoner 

 metals together with the resistance per metre and the length per ohm for wires of one mm diameter, also 

 the relative resistances. In addition there is also a supplementary rheostat consisting of >/i metre of 0.3 mm 

 thick platinum, iron, brass, and copper wire and 1 m each constantan wire 0.3 and 0.6 mm thick and man- 

 ganin wire 0.3 and 0.9 mm thick. All these wires are accessible singly. 



All numbers are painted on very distinctly and are visible at a distance. 



in. XT 1. - idem, without supplementary resistance and without table of resistances 



on the back 6. 0. o 



61,875. Demonstration Sliding and Plug Rheostat, after Krlemann, Figs. A, B and (.' 



(Ztschr. f. d. phys. u. chem. U. 22, 1909, p. 353) 12. 0.0 



The rheostat consists of two upright boards joined together by a hinge and a single board for the 

 single ohm, it being possible to attach this smaller board to the large one. Fig. A shows the front with 

 the single ohm 1 ohm attached, plug resistance 210 ohms, and sliding resistance o In ohms. A 

 copper, iron and Herman silver wire of equal length and equal cross section are stretched zig-zag fashion 

 across the back (Fig. B). In addition the back contains s \\ire- of :>(> cm length (3 each of cross Section 1. 

 2 of cross section -J. ami :; of cross section 3). Fig. C shows the front folded up with the single ohm taken 

 off and fixed on a special baseboard. 



This rheostat can be used in a great variety of ways and the zig-zag arrangement of the resistances 

 assists the comprehension of the students. 



f.l.XTti. - - idem, with back blank 10.10.0 



M. 5179,5180. 



