3451143

Information

• Patent Grant
• 3451143

• References
• Source

• Patent Number

  3,451,143

• Date Filed

  Not available

• Date Issued

  Tuesday, June 24, 1969
  55 years ago

• CPC

  • G09B3/085 - with invisibly printed correct answer; the answer being rendered visible by special optical means
  • B41M5/0253 - using a chemical colour-forming ink
  • B41M5/04 - using solvent-soluble dyestuffs on the master sheets

• US Classifications

  • 434 - Education and demonstration
  • 101 - Printing