23° 



SCIENTIFIC NEV\^S. 



[Dec. 1st, if 



annular reservoir is thus formed, and the thin layer of mer- 

 cury it encloses is very susceptible to the changes of tem- 

 perature of its surroundings. Moreover, the errors due to 

 the expansion of the glass are reduced practically to zero, 

 as the two glass cylinders expand in the same direction. 

 The principle here introduced appears to us a sound one, 

 and might be applied with advantage to thermometers 

 other than clinical, as, for instance, those used for delicate 

 experiments in thermo-chemistry. A reading microscope 

 can be attached to the stem of the thermometer (as shown 

 in Fig. i), and as the graduations in these thermometers are 

 very fine, this is a very convenient addition, especially as 

 the microscope, when not used, can be fitted on to the end 

 of the thermometer. 



THE TELEPHONE : ITS PRINCIPLES, 

 CONSTRUCTION, & APPLICATION.— V. 



But the most important use of the telephone is in connec- 

 tion with the exchange system. The object of this, as 

 most people know, is to enable anj'' subscriber to an ex- 

 change to communicate with any other subscriber. The 

 commercial and social advantages of such a system are evi- 

 dent, and it is found that although it is a little difficult to 

 get people to pay for them when starting an exchange, a 

 few months' experience teaches the most sceptical that the 

 service is well worth its price. The arrangements just de- 

 scribed are widely used for communication between diffe- 

 rent establishments belonging to the same firm, between 

 the business and the residential premises of professional 

 and commercial men, and in other similar circumstances, 

 but there are few cases in which two firms have sufficient 

 business with each other to justify the expense of a special 

 communication, and it is obviously impracticable for every 

 large firm in London or Manchester to have a special wire 

 to all the firms with whom it has dealings. The Exchange 

 system gives to each subscriber the power of communica- 

 tion with any other subscriber at the cost of only one line 

 and set of instruments. The arrangements are simple in 

 principle, but become intricate in practice. Each subscriber 

 is provided with a set of instruments, such as has just been 

 described, consisting of a transmitter, a receiver, a bell, and 

 a ringing-key and battery, or magnet machine, connected 

 with a line wire running to the exchange. At the ex- 

 change each line wire is connected to a switchboard, which 

 is under the observation of an operator. As a rule, each 

 switchboard takes fifty to one hundred lines, so that in a 

 large exchange there will be several boards, and as many 

 operators on duty. Each wire has allotted to it an indica- 

 tor and a simple means of making a connection to it by 

 pushing a plug or jack into a hole or slot on the board. 

 The operator is also furnished with a set of speaking and 

 ringing apparatus, which she (telephone operators are 

 mostly girls) can put into connection with any line at will, 

 and also with a number of flexible wire-conductors, each 

 furnished with a plug or jack at each end, to enable her to 

 connect any two lines together. The indicator and the con- 

 necting-hole allotted to each line are marked with the sub- 

 scriber's number in the telephone list. When a line is not 

 in use, it passes through the indicator and to earth, through 

 a spring which presses against another one connected to the 

 earth. These two springs are opposite to the hole or slot 

 in the board, and when a plug or jack is inserted they are 

 separated and insulated from each other. The indicator con- 

 sists of an electro-magnet, which, when excited by a current 

 passing through it, allows a little shutter to fz'l and reveal 

 the subscriber's number in bold figures. Sometimes the 

 fall of the shutter is made to ring an electric bell, but this 



is not necessary with attentive operators, and is rather a 

 source of distraction and annoyance in a busy exchange. 

 The following is the routine of connecting two subscribers 

 together. Suppose that number 213 wishes to speak to 

 number 250; 212 works his ringing apparatus, and the 

 operator signifies her attention by ringing his bell. 212 

 then takes his receiver off the hook, and says, " Connect 

 me to 2 5 o." The operator repeats the figures back, and 

 212 then hangs up his receiver. In a minute or two his 

 bell rings again, and taking down his receiver, he finds 

 himself in communication with 250. On the completion 

 of their conversation, both hang up their receivers, and 

 work their ringing-key or magneto. The operator, on per- 

 ceiving that indicator shutter 212 has fallen, puts a plug 

 into No. 212 hole, which plug disconnects line 212 from 

 earth, and conne:ts it to her speaking and ringing appara- 

 tus. She then rings back to 212, and immediately after- 

 wards lifts her receiver to her ear ; or, if it is carried on an 

 ear-high stand (as is usual to save time and fatigue), switches 

 her speaking apparatus into the circuit, and hears 212 ask 

 for connection to 250. She repeats back the number asked 

 for, and if 250 line is not in use, which the can see at a 

 glance, connects her apparatus to 250, rings him up, and as 

 soon as she gains his attention, connects the two lines to- 

 gether by means of one of the flexible cords, first giving 212 

 a ring up to notify him that he is connected. When the 

 conversation is completed, the ringing current makes an in- 

 dicator shutter in circuit with the flexible connection-drop, 

 and the operator withdraws the plugs and leaves the two 

 lines free. In practice, a number of switches are in use, 

 which enable the operator to readily connect either her 

 ringing or speaking apparatus to each line which is calling 

 or being called, or to interpolate her apparatus between two 

 lines which are in use, and break in upon the conversation 

 with a hasty " Have you finished ? Have you finished ? " 

 which is sometimes very provoking. The illustration sup- 

 pose that both lines are on one board and under the con- 

 trol of one operator, but in large exchanges there are a 

 number of boards. The different boards are connected to- 

 gether by short lines, each with its indicator and connecting- 

 hole, and if a subscriber connected to one board wants a 

 subscriber connected to another, the operator controlling the 

 board on which the call is received has to ask the operator 

 at the board to which the required line is connected to con- 

 nect that line to one of the lines between the two boards ; 

 and the first operator then proceeds exactly as if that line to 

 the other board were the line she requires, as of course it 

 practically is. In London and some other large towns, dis- 

 tance, the number of subscribers, and the difficulties in the 

 way of bringing a very large number of wires to one centre, 

 render it necessary to have a number of exchanges scattered 

 over the town or district. These are connected together by 

 " trunk lines," and every operator knows at once, on being 

 asked for a connection to a given number, what exchange it 

 is on, a certain range of numbers being allotted to each ex- 

 change. When any subscriber on one exchange wishes to 

 communicate with a subscriber on another, the operator has 

 to get the required line " put through " to her, in much the 

 same way as if it were merely on another board in the same 

 exchange. It will be easily seen that when several hundred 

 lines have to be provided for, the number of cross-connec- 

 tions becomes very large. As simplicity of working is the 

 great object aimed at, in order to economise time and pre- 

 vent misconnections as far as possible, and as simplicity of 

 working involves complication in the apparatus, the neces- 

 sary connections become very intricate indeed. In a tele- 

 phone exchange of the first order, the number of separate 

 connections amounts to hundreds of thousands, and many 

 miles of insulated wire are employed in making them. 



