110 Experimental Switchboards and Experimental Resistances. 



resistances and the marble: these layers consisting of asbestos and air. The asbestos strata hold off 

 the heat from the slab, and the air-strata, being heated by the heat given off by the asbestos, take 

 an upward direction, thereby effecting good ventilation. But the intermediate layers of asbestos 

 fulfil another purpose: the leads from the switch contact are carried between two jof them to the 

 individual resistance spirals. Thus a number of undesirable wire crossings immediately behind the 

 switchboard are obviated, and under these conditions the resistance spirals cannot accidentally come 

 into contact with the connecting leads. The factor of safety of these new switchboards is therefore 

 essentially higher than in the old construction. 



. As to switchgear and measuring instruments for each circuit, there are: 1 two- or three-pole 

 fuse, 1 main switch, one double connection switch for regulating the current, 1 switch for the slnint, 

 1 change-over switch for measuring the potential in the supply line and at the apparatus, 1 change- 

 over switch for measuring the current in the main circuit and in the shunt (this switch and the 

 measuring resistance pertaining to it are only employed in connection with switchboards which are fitted 

 with precision instruments), 2 measuring resistances (shunts) to enable these current measurements to 

 be made one after the other without interrupting the current; 1 plug box for making connection with 

 the apparatus to be tested; 1 ammeter and 1 voltmeter. Connection to the supply line is in the case 

 of the stationary switchboards, made by means of terminals; and in the travelling and portable types 

 of board, a flexible lead terminating in a plug is used for the purpose. This plug is inserted in plug 

 boxes fitted at the places where it is desired to use the board. The measuiing instruments priced 

 in the list differ in size and construction in the various switchboards, and in some cases instruments 

 with 2 ranges are employed. All these circumstances must be duly taken into consideration in jud- 

 ging of the price. 



Arrangement of Connections. 



Method I. Rheostat as Series Resistance. 



This arrangement can be employed with advantage for the charging of accumulators, the 

 excitation of electro-magnets, and for working an arc lamp up to the current capacity of which the 

 switchboard permits. The illustration appended shows the scheme of connections. 



Switch for Shunt open. -- Switch contact I of the double switch contact is on the last contact 

 to the right, and switch contact (or lever) II on 0. The current starting from +. takes the path 

 indicated in the figure by thick lines and arrows: positive pole 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, plug 

 box 12 (the experimental apparatus is connected to this), 13, 14, lever II of the double switch con- 

 tact, resistance 15. 16, 17, lever I, 18, 19, 20, 21, 22, 23, negative pole. The diagram shows, in ad- 

 dition, by dotted lines, the connection of the shunt leads at 6, 7, 9, 10 to the ammeter switch, and 

 at 5 and 19 and 11 and 13 respectively to the voltmeter switch (and from these switches to the mea- 

 suring instruments themselves). The switch for the voltmeter is placed on "Netzspannung" (network 

 voltage), and the ammeter switch on "Stromstarke im Apparat" (current in the apparatus). By 

 changing over the latter to "Gesamtstromstarke" (total current) will, with this method of connecting, 

 give the same deflection of the pointer. 



In this method of connections the resistance inserted is always equal to the difference between 

 the numbers placed alongside the contact pieces on which the lever is placed. At the position of 

 the lever indicated in Diagram I the resistance in circuit is the total resistance of the double con- 

 tact lever, e. g., by employing a Type Ai switchboard as listed, it is 36 ohms. If the plug box is 

 short circuited by connecting the contacts of same with a terminal, we have, with the position of tin- 

 lever given: 



with Type A and 110 Volts working pressure a current of = 3.05 amperes; 



3o 



1 i * i \ 



with 160 Volts working pressure = 4.44 amperes; 



oo 



with Type B and 220 Volts working pressure "'"I = 4.58 amperes; 

 with Type C and 110 A'olts working pressure = 4.58 amperes. 



') In The ease of all switchboards not constructed with a change-over switch for current iir-:isnrcm<-nts and 

 with measuring resistances, the ammeter remains permanently in circuit. 



