THE GEAMOPHONE EEDDIE. 223 



nears the center of the blank, but there then arises the necessity of 

 using mechanism for securing a corresponding gradual change of 

 speed on the reproducing machine in order to keep the selection in the 

 proper key. Devices for securing an increasing speed have been 

 invented, but they are not free from objection, and have never come 

 into general use. 



The record in wax having been made, the next step is to produce a 

 negative in copper. The wax tablet is dusted with graphite, which 

 is worked into the grooves with a badger-hair brush, to make it elec- 

 tro-conductive, and is lowered into the electrolytic bath of copper salt 

 solution. In order that this negative may be able to resist the pressure 

 to which it is subjected in pressing records, ;t is necessary that the 

 deposition of the copper should be thoroughly homogeneous. To this 

 end, and also in order to hasten the process so that the blank may not 

 be attacked by the solution, the blank is kept continuously in motion 

 in the electrolytic bath. The process is continued until the copper 

 shell is nearly 0.9 of a millimeter in thickness. The negative thus 

 formed may be termed the master negative, and from this master a 

 few commercial samples of the record can be pressed by means of 

 which the quality of the record can be tested. It is not, however, 

 usual to press more than two or three records from this negative. 

 Seeing that sometimes as many as six thousand or more copies are sold 

 of a single record, it is natural that the manufacturers should take 

 steps to enable them to multiply copies without injuring their master 

 negative or having it worn out, for it is not usual at this stage to 

 obtain further negatives from the original wax record. They accord- 

 ingly make duplicates of their master negative, by taking dubs or 

 impresses of the master in a wax composition, from which in turn 

 working matrices are made. Copper shells are obtained from these 

 dubs in the same way as from the original wax tablet, but the metal 

 is only deposited to the thickness of about half a millimeter. The 

 shells are made absolutely true and flat at the back, so that any irregu- 

 larities caused in the electro-deposition may not be transferred in 

 pressing to the front or face of the shell. They are then backed up 

 or stiffened by a brass plate about one-tenth of an inch in thickness. 

 The attachment of the backing plate and matrix is effected by sweat- 

 ing or soldering them together under pressure. The backing plate 

 is supported on a heated table, a thin layer of solder is run over it, 

 the shell is laid upon it and pressed firmly down, with an elastic pro- 

 tective cushion of asbestos, for example, placed over the face or re- 

 corded svirface of the shell to prevent the sound ridges in it from 

 being injured. The matrix thus obtained is now nickel plated on 

 the recorded side so as to present a better wearing surface, and after 

 polishing is ready for use in the pressing machine. 



