178 SMITH: T E L E P H O N E I N T E R F E R E N C E Journal Α. I. Ε. E

equal to, or less than, that found in the best commercial practise in machine design. A study of the magnitude of these harmonics seems to show that none of the ordinary methods open to- a designer could be used to reduce these harmonics with certainty to much smaller values. If telephone noise is caused by such a machine, the question of using means for grounding external to the machine should be considered, and whether general conditions will warrant radical modifications in machine design as opposed to other methods of wave correction external to the machine.

METHODS WHICH HAVE BEEN USED TO MINIMIZE THIS TYPE OF INTERFERENCE

In the complete paper, references are given to articles already published describing various methods which have been used to relieve such situations. Some of the methods described apply to generators already installed and which are found to give rise to interference; others refer to certain points which, if given advance consideration, may avoid later modifications if trouble should develop.

CONCLUSIONS

The difference in behavior of the triple harmonics and non-triple harmonics has been illustrated, and it has been pointed out how relatively small amounts of triple harmonic voltage existing from line-to-neutral may give rise to more noise on paralleling telephone circuits than considerably larger amounts of non-triple har­monic voltages.

A method has been suggested by which separate measurements can be made of the triple harmonics alone, by a special connection of the machine for test. These measurements can be combined, if desired, under certain assumptions which have been given, with the measurements made in the usual way so as to give an extra weighting to the triple harmonic series which would correspond to their greater inductive effect due to the fact that they return through the earth connection.

A number of references to methods which have been used to minimize this type of interference have been given. Some of these are applicable directly to the design of the generator and others involve auxiliary apparatus for the reduction of the harmonics. A little advance consideration of the subject of this paper may sometimes avoid later modifications if trouble develops.

Although the majority of systems in this country at the present time are operating with grounded neutral, the actual cases of this type of telephone interference are relatively few. Many generators, of course, are connected to transmission lines through delta-Y trans­formers which greatly reduced the possibility of inter­ference due to the triple harmonics. The cases which have occurred have been serious enough, however, to create a real problem.

There are so many factors involved,—such as the type of power system, proximity to and length of

parallels with telephone circuits, capacitance to ground of power conductors beyond parallel, etc.,—that it would be extremely difficult to predict beforehand in any given location whether or not this type of telephone interference will arise. In some cases in which a ma­chine gave rise to telephone interference on one system, it has been found that a duplicate machine operated for years in another location without giving any trouble. This type of interference has occurred more frequently in the last few years, although for many years, it has been common practise to ground systems and although the wave shape of these large turbo alternators has been gradually improved. I t can be thus seen that the problem is quite complicated and worthy of study by operating as well as designing engineers.

BUILDING WITHOUT WINDOWS Experiments are being conducted, according to

George F. Paul of Chicago, to determine whether some better method than windows can be devised to utilize natural sunlight for the interior of buildings. There has been established in Chicago a windowless experi­mental laboratory. This laboratory is flooded with sunlight and sky shine by means of small bull's-eyes that are located along the outer wall and near the ceiling to replace the usual windows.

Dr. Frank E. Hartman, noted Chicago physicist, has designed this unique laboratory. He declares that windows are wasteful of heat, that they distract the attention of workers in offices, that they are inefficient for ventilation, and that they are a costly part of any structure.

"Few schoolrooms and practically no large offices and workshops can be uniformly illuminated by windows," declares Dr. Hartman. He says that much of the complaint of improper heating in winter is due to the disturbing influence of glass, and that it is difficult to locate direct radiation effectively when large glass surfaces are encountered.

The window is, after all, more a necessity of custom than of fact, according to Dr. Hartman. He calls attention to the fact that they are located in vertical walls and that during most of the day they receive only diffused or reflected light. Furthermore, due to the width of the streets and the height of buildings, only the topmost windows derive any advantage from the sun during the morning and afternoon periods.

Another disadvantage that Dr. Hartman finds is that most window glass shuts out the sun's ultra-violet rays The ordinary street offers no reflecting sur­faces suitable for directing sky shine into windows. On the contrary, it offers surfaces of tremendous absorbing power, with the result that practically no sky shine enters windows.

Dr. Hartman says finally that he believes that the sky shine can best be made available indoors by the use of a series of metallic concentrators, condensers, and reflectors.—Professional Engineer.