INS ULA TORS. 
121 
When it is required to interrupt a wire or two at an office, 
the insulator called the interrujotor is used. This is represented 
in fig. 57. As it is of small size, it can easily be fixed by 
means of square irons as in fig. 59, or by double irons (fig. 60) 
against a plank or any flat wooden surface. 
The tenon iron, shown in fig. 61, is intended for fixing in 
stone walls. The hook iron (fig. 62) allows an office to be 
thrown into the line wire without the necessity of fixing the 
support against a post or a building ; for one end of the line 
wire is attached to the hook and the other to the interrupter 
carried by the iron. As this interruptor gives only a partial 
insulation, it can only be used in places sheltered from 
moisture, or on lines of short extent. 
The insulators called fiat, closed, and open insulators (figs. 
59 and 62) which were formerly used to support the wires 
