Some technical details are given in an article in Scientific American, 21 Mar. 1903, p. 206.
The use of pneumatic tubes in transmitting money, papers, and parcels of various kinds has become so extensive that the service is considered a necessity in the equipment of the modern mercantile establishment. The plan is not a new one. Improvements, however, which have been made in the system in the last two or three years have greatly increased its practical value. In the dry goods or department store, for example, it is valuable as a labor saver, dispensing, as it does, with the many cash boys, in some instances cash girls, that have been employed , and performing their work much more quickly, besides avoiding many mistakes which formerly occurred. There is no delay in " making change," as the amount due the customer is usually handed him by the salesman within a minute, some times less than a minute, after his money has been taken over the counter. The system also assists in checking or auditing the sales, for the charge or cash slip which represents the amount of the transaction is sent to the cashier or bookkeeper, where it is examined and verified before being returned.
In the ordinary store the pneumatic tubes extend
from the cashier's and bookkeeper's departments to
the principal sales departments, varying, of course, in
number according to the extent of the establi shment.
Each tube is termed a "line" and is usually 2 1/4
inches in diameter. The tubes are generally extended
along the ceiling or under the floors for the purpose
of economizing space, and the terminals where the
carriers are received and sent are of various shapes
adjusted to suit the conditions. The system is so laid
out that when a sale is made the clerk prepares his
purchase check, gets the money from the purchaser,
and places it in a small brass cylinder which can be
unscrewed at the end merely by a twist of the fingers.
To start the carrier, it is necessary only to insert it in
the receiving end . The air forces it through the line
to what is called the main station. This is usually
in the cashier's office, for so many articles in the retail store are sold for cash that no entry is required.
The carrier drops into the open receiver at the end of
the tube, from which it is taken by the "change
maker," who, as already stated, glances over the figures on the slip and verifies the total. If an error has
been made, the slip and money are returned to the department from which they were sent. If correct, the
slip is returned with the amount due the customer. If
the sale is to be charged, the slip of course contains
the name of the customer in addition to a description
of the article and the amount due. As soon as it has
been examined, the clerk in the cashier's office again
places it in the carrier and inserts it in the tube or
line connected with the bookkeeper's department. Here
the memorandum is taken out, entered on the books,
and either the original slip or a duplicate is returned
to the salesman.
These operations are usually performed in less time
than it takes to read the description; for the carrier
travels at a rate varying from 1,000 to 2,500 feet per minute, according to the air current. The length of a
line is seldom over 600 feet. The current is produced
by the blower system, and the mechanical plant installed provides for a force representing from one
fourth to one-half horse power to each line, depending upon the number of bends or curves and the
amount of service. A store having a "50-line" service
therefore requires an engine of about twenty-five horse
power. In some systems the blowers are operated by
steam power direct, but electric motors, either direct
connected or bolted to the blowers, are preferred.
The air current is maintained in the tube system in the following manner : The various lines are connected with what may be called a main conduit, which leads to the engine room and to the blowers. These blowers draw the air from the various sending terminals of the line, expelling it through a conduit of suitable size, which may open in the engine room or be connected with the street. While the velocity of the current varies according to the speed of the blower fans, the minimum is rarely less than 2,500 feet per minute, the pressure in the tubes ranging from 6 to 12 ounces per square inch, the latter pressure being secured with a service of one-half horse power. The principle is simply the exhaustion of the air in the tubes to produce a partial vacuum. The effect is so powerful that, although the carriers and their contents weigh a half pound, they are transported without difficulty. The suction is not apparent twelve inches from the end of the receiver. Consequently, the end of the receiver can be placed over a desk or table on which light material, such as paper or currency, is spread. Incidentally the system is of considerable value from a hygienic standpoint, as it assists in the ventilation by continually changing the air in the apartment where the terminals are installed.
The carriers are merely cylinders of sheet brass covered at each end with felt to protect the metal from
abrasion in passing around the elbows of the tube.
They range from four to six inches in length for the
ordinary store service, but do not fit closely against
the side of the tube. Enough space is provided to allow the carrier to be borne along by the air current
with little or no friction except at the turns, thereby
permitting of a much greater speed than if the carrier acted as a piston and was continually in contact
with the tubing. The receiving terminals are of two
kinds, although both work automatically. The ones
used in the cashier's and bookkeeper's department are
merely open tubes, which are usually suspended over
a receiving table or desk. An air valve is placed in
the receiver at a point three or four feet from its end.
This is so adjusted that merely the pressure of the
carrier against it opens the valve. The carrier then
drops by gravity to the end of the receiver, and is
taken out by the cashier's clerk or bookkeeper. As
soon as the carrier passes, the valve is shut by a spring,
and thus the current is confined. The air is then diverted into a parallel tube connected with the sending
terminal, the operation of which has already been described. The return tube to the sales department also
terminates in the valve, which is located directly at its
mouth. When the carrier is sent back, its impact is
sufficient to open this valve, and it drops upon the
salesman's table, the valve closing automatically and
confining the air current as in the other instances.
The system in the cashier's and bookkeeper's department requires some one to take the carriers out, in
order that they may be examined as they are received, thus preventing unnecessary delay in making
change. As the extent of the service is limited only
by the power of the blower plant, some of the pneumatic systems which have been installed in department
stores recently constructed are very extensive. Perhaps the largest in the United States is located in
Philadelphia. It consists of over 250 stations, each
connected with a line varying from 400 to 500 feet in
length. A plant of 150 horse power is utilized, and in
all nearly 20 miles of tubes are used. The power is
sufficient to force the carriers through every line
as rapidly as they can be inserted in the tubes.