The size and quality of the iron core, the relation between the number of turns in the primary and secondary windings, and the mechanical construction of the induction coil are matters of the greatest importance, and have not in general received the attention which they merit.

A number of attempts have been made to calculate mathematically the best dimensions and resistances of the telephone induction coil, but the matter is of such an extremely complex nature, and all of the quantities are subject to such complex and indeterminate variations, that the results so far produced have been in general unreliable.

Only a few series of experiments are on record from reliable sources giving the results of comparative tests between induction coils of various dimensions.

It may be said that definite results from any such series of tests are extremely hard to get, as the quality and loudness of transmission is subject to a very great personal error, even in the case of experienced experimenters.

In making comparative tests of any telephone apparatus it is of the greatest importance that all possibility of prejudice on the part of the experimenter be removed, and in order to do this it is essential that he be in ignorance at all times of the particular instrument that he is testing. To illustrate this point, suppose that three transmitters are being compared with respect to determining the general talking qualities of each.

If the party at the receiving telephone, who is to judge of the hearing, desires that one of these instruments produces better results than the others, he is very sure, even though he be strictly honest with himself, to conclude at the end that that transmitter is by far the best ; unless, of course, there is a very marked difference between them.

For this reason he should be kept in ignorance of the particular transmitter in circuit with the line at any time, and should only be told when changes are made. It is well to have the instruments numbered, the party judging of the merits to be in ignorance of the transmitters to which those numbers refer.

It is also a somewhat difficult matter in comparing the clearness with which instruments transmit or reproduce speech to select proper subject matter to be transmitted.

It is unfair to the first instrument tested to repeat the same sentence or read the same matter in each case, for the reason that the listening party becomes more or less familiar with the matter to which he is listening, and therefore often catches words at the second or third reading which he fails to grasp at the first.

It is therefore better to read a selection from a certain article in the first test, and a continuation of the same matter in each successive test, so that the character of the matter read will be approximately the same in each case.

However, where several instruments are apparently of almost the same merit, and where the transmission is so good that all of the matter may be generally understood, it has been found that a better way is to prearrange a number of series of words, a different series to be read into each instrument under test. Care must be taken, in the selection of these words, that each series contains words of the same character.

To illustrate : suppose five instruments are to be tested. Five different series of words may be prepared, containing such words as the following :

sign rhyme dine fine mine

going rowing sewing mowing throwing

missile thistle whistle bristle gristle

etc. etc. etc. etc. etc.

Each list should contain about forty words ; and as the words in each series will be seen to differ from the corresponding words in the others to only a slight extent, no instrument can be said to have an easier list than the others. The first list should be read into the first instrument slowly enough for the receiving party to write them down. Then the second list is read to the second instrument, and so on.

Such words are difficult to distinguish, especially when there is no context, as is the case in reading an arbitrary list of words. The receiving party should be required to write down the words as he hears them, and the list which is most correct according to his notes will probably represent the work of the best instrument so far as clearness is concerned.

It is only by a consideration of such details as these, simple though they be, that an unbiased opinion can be formed as to the relative merits of telephone equipment.

Many elaborate experiments have been performed for arriving at the comparative merits of similar instruments depending on the quantitative measurements of the amplitude of vibrations, the amount of current, and similar quantities, but in the first place the apparatus and time for such measurements are available to but few, and in the second place it is doubtful if the results obtained are as reliable as those obtained by carefully following the above suggestions carefully.

By following these experiments carefully, telephone designers are able to make sure that we can understand each other clearly when making or receiving calls!