PLATED QUARTZ CRYSTAL UNITS 245 



result. The gap in then tested for presence of metallic particles by grad- 

 ually impressing voltages up to 1000 volts a-c across it. If no fiashover 

 occurs the division is satisfactory. If flashover occurs the voltage is 

 maintained until the slivers are burned out. The hypo and burning treat- 

 ments are repeated until a good division is obtained. Figure 13.5 shows an 

 electric dividing tool developed for this purpose. In using this method it 

 has been found that the arc at the point of the stylus may cause twinning 

 of the quartz to a minor extent along the dividing hne. This effect is 

 usually insignificant although it may be objectionable especially where pre- 

 cise values of crystal inductance or frequency-temperature performance are 

 required. Where more complicated divisions are necessary, as in the case 

 of face flexure and harmonic plates, the electric stylus method is employed, 

 although methods and tools for performing this operation by other means 

 to avoid twinning are being developed. 



13.44 Attachment of Wire Supporting Leads 



Phosphor-bronze wire is employed in wire supported crystal units pri- 

 marily because of its high tensile strength, and excellent fatigue resistance 

 characteristics. Five- and six-mil diameter wires are the most widely used 

 sizes, depending upon the mass of the crystal plate, the desired electrical 

 performance, and the severity of treatment it is Hkely to encounter in use. 

 To facihtate soldering the wires to the spot on the crystal plate and to the 

 crystal support system the phosphor-bronze wire is given a heavy electro- 

 tinned finish. 59.5-34.5 per cent tin-lead eutectic solder saturated with 

 approximately 6 per cent silver at 570''F is employed for attaching these 

 fine wires to the silver spots of the crystals. This solder solidifies at approxi- 

 mately 360°F with practically no mushy stage. The reason for saturating 

 the solder with silver is to discourage migration of the silver in the spot to 

 the solder during the soldering operation. Even with this solder it is advis- 

 able to limit the time for heating of the joint to a minimum. 



One method of attaching the wires to the crystal plate is by means of a 

 special machine developed for the purpose. Such a machine is illustrated 

 in the photograph on Fig. 13.6. The wire is fed from a spool through the 

 head in the movable arm. The head contains a wire guide having a hole 

 only slightly larger than the diameter of the wire and a small vise for firmly 

 clamping the wire. The crystal plate is clamped in the vise on the hot plate 

 which is thermostatically controlled at approximately 240°F. The position 

 of the arm carrying the wire is lined up with respect to the crystal plate 

 by means of guides so that the wire will be placed exactly on the nodal point 

 or line of the crystal. In making the attachment, with everything fined up, 

 the wire is fed through the guide until it touches the spot on the plate and 

 the vise closed. Since the curvature of the wire can never be entirely 



