Type of current and construction of the switchboards for experiments. 109 



Construction. 



The switchboards are substantially constructed in an expert manner, all boards being con- 

 structed to the Safety Specifications of the German Society of Electrical Engineers; they can there- 

 fore be connected up at will to any heavy current main. AVhite marble solely is used for the slabs. 



The switchboards are supplied in three patterns, viz., as wall boards, portable, or of the 

 travelling type. In the Wall Type the resistances are placed in an iron housing which is screwed 

 firmly to the wall, care being taken to ensure good ventilation. The housing has a marble slab in 

 front on which are mounted the various switches and measuring instruments (see No. 50,411). This 

 arrangement has the advantage that the experimental switchboard does not take up a large amount 

 of wall surface. If desired, or if the local conditions warrant it, the resistance is arranged separate 

 from the switchboard and connected thereto by a number of leads (see Nos. 50,477 and 50,504). 

 The Portable Type can only be recommended for switchboards with but one circuit, otherwise the 

 resultant board is too heavy. In construction it is similar to the wall boards, with the difference 

 that it rests on feet and has two handles (see Fig. 50,471 A). This switchboard can be removed 

 from the lecture room when it is desired that the scholars' attention should not be drawn to it; 

 and in laboratories the switchboard can be used in any spot desired, it being only necessary to have 

 at the places in question a plug box to take the plug connected with the switchboard. The Travel- 

 ling Type possesses the advantages just mentioned but in a higher degree. These switchboards can 

 be constructed with two circuits and they can be manipulated by a single person. These boards 

 are in table form, the resistances being contained in a box-shaped lower portion, and the measuring 

 instruments and .switches being mounted on a marble slab placed above this lower part (see No. 50,472). 

 The instruments are mounted in a sloping position on iron supports so as to be easily read. The 



a experimental switch-table rims on rollers having rubber tyres, the rollers turning laterally and there- 

 fore always being in the direction of travel. The resistance can also, together with the double switch 

 contacts, be separated from the switchboard and made of the travelling type as shown in Fig. 50,471 B. 



O In this form of construction the resistance can be used without the switchboard as a series or regulating 

 ' resistance in the laboratory or preparation room. This construction will be considered in cases where 



tS the resistance when not being used should take up no space. Only 3 leads need be laid between the 

 switchboard and the resistance. 



S 



g The demands put upon the experimental switchboard are very great, for the highest possible 



current-density is desired as well as a regulation in very close stages. This circumstance must be 

 & taken into account both in the construction of the regulating device and the dimensioning of the 

 ^ resistances. The regulating apparatus is therefore constructed in the Weinhold Double Switch Contact 

 _ form. The double switch contact arrangement consists of contact pieces arranged in a circle, upon 

 which slide the contact springs of two levers insulated from each other. The figures at the side of 

 W the contact pieces give the resistance in ohms between one contact piece and the contact piece 0. 

 * The handles of the levers are placed laterally on the levers themselves, so that both switch contacts 

 (or level's) can be placed on the same contact piece. This switch permits (1) the- employment of the 

 resistances in series with the apparatus; (2) one portion of the resistance being put in series with 

 the apparatus to be tested and another portion in parallel with the same, i. e., in shunt, thus ob- 

 taining a division of tension. 



By this arrangement alone a multiplicity of positions in the regulating stages is attained, 

 and this can be made use of in its entirety for instructional purposes and in the laboratory, as the 

 transition from one form of grouping to another in the arrangement which we have adopted can be 

 carried out with great rapidity by simply cutting a single switch in and out of the circuit. The 

 resistance is connected up as a series resistance when the shunt switch is open, and as a shunt resis- 

 tance when the shunt switch is closed. By employing a large number of contacts (30 in the ordinary 

 and 21 in the simple type of construction) the graduation is brought to a suitable degree of fineness. 

 Another important advantage is secured by the adoption of the double contact lever arrangement, 

 vi/., the possibility of securing coarse regulation with one handle and fine with the other; this has 

 proved to be of great value, and, indeed, almost indispensable, for educational purposes and in the 

 laboratory. If it be desired to keep the fineness of regulation within still narrower limits, a Sliding 

 Eheostat is put in series with the apparatus to be tested; this type of resistance is especially desirable 

 in laboratories. The resistances belonging to the switchboards are dimensioned to correspond to the 

 wide range of regulation which is always demanded of experimental switchboards. By organically 

 combining the resistances with the switchboard to which they pertain, it is not generally necessary 

 to have a special room for the resistances. The resistances are designed for continuous loading with 

 the currents stated in the price list, when the correct contacts are used. When a continuous and 

 heavy load is applied to the resistances, a fairly large amount of heat is radiated from them on to 

 the back of the marble slab. 



As such considerable heating on one side may under certain conditions be deleterious to the 

 marble slab, heat-insulating layers are, by a new and practical arrangement, placed between the 



